Disc Brake System Inspection and Service

Transcription

1 C H A P T E R 1 5 Disc Brake System Inspection and Service Chapter Objectives At the conclusion of this chapter you should be able to: KEY TERMS disc brake micrometer on-car lathe parallelism pulsation rough cut runout taper thickness variation Copyright 201 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the ebook and/or echapter(s).

2 362 Chapter 15 Disc Brake System Inspection and Service Disc brake service, while usually not very difficult, does require that you take your time to thoroughly inspect the brake system and understand all of the details necessary for proper service and repair of the system. Performing a brake job means more than just installing a set of brake pads, especially if diagnosing a noise or pulsation complaint is required. Service Tools and Safety Performing brake system services, such as brake pad replacement, is among the most common work technicians perform. Most employers expect that entrylevel technicians can correctly diagnose and service disc brakes. While pad replacement is, in itself, an easy task, there is more to disc brake service to ensure that the brakes perform properly and provide the noise- and vibration-free operation that the customer expects. Tools As with all repairs, the proper use of tools and procedures is an important aspect of performing your work safely and efficiently. Improper use of tools and taking shortcuts can lead not only to damage to the vehicle but can also cause personal injury. Always use the proper tools for a particular service or repair, and follow all manufacturer service procedures when you are working on the brake system. While there are no unimportant parts on a vehicle, the components of the brake system are, arguably, among the most important parts of any car or truck. Disc Brake Service Tools. The most commonly used tools for brake service are your basic hand tools. Brake calipers are attached with hex-head bolts, Allen or Torx head bolts, and most other components use either standard or metric nuts and bolts. Specialty tools for brake service include the following: Lining wear gauges, as shown in Figure 15-1, are used to measure the amount of lining material left on the brake pads. These gauges allow you to measure lining thickness with the pads still installed on the vehicle. In place of these, a tire tread depth gauge or machinist ruler can be used once the pads are removed. A piston retraction tool, like that shown in Figure 15-2, is used to push the brake caliper piston back into its bore. The piston retraction tool shown in Figure 15-3 is used on rear disc brake systems that use a parking brake integral with the rear caliper. Seal tools, like those shown in Figure 15-4, are used to install seals when rebuilding calipers. FIGURE 15-1 Brake wear gauges are used to determine the amount of lining left on pads and shoes. FIGURE 15-2 This tool retracts the caliper piston back into the bore. FIGURE 15-3 This tool retracts the caliper piston on integral parking brake calipers.

3 Chapter 15 Disc Brake System Inspection and Service 363 also observe the follow disc brake system service precautions: Dust boot installer Dust boot pliers FIGURE 15-4 These tools are used to install caliper seals when rebuilding a caliper. To accurately measure brake rotor wear, a disc brake micrometer like that shown in Figure 15-5 is used. To measure runout, a dial indicator like that shown in Figure 15-6 is used. In addition to basic hand tools and those listed above, you also use both on-car and off-car brake lathes, which are discussed later in this chapter. Brake Service Precautions. Aside from using the correct tools properly for brake service, you should FIGURE 15-5 An example of a rotor micrometer. Note the pointed anvil for reaching into grooves to get the most accurate reading. FIGURE 15-6 A dial indicator is used to measure rotor and hub runout. Let the brakes cool before you service the components. Use calibrated torque wrenches when you tighten the fasteners. Do not let the brake calipers hang by their hoses. Safety Even though most brake services are routine, do not take for granted the work that you perform. When you are working on the vehicle, remember to do the following: Double-check the lift or jack contacts before you begin your work. Do not use your back muscles to lift when you are removing and installing the wheels and tires. Use your legs or ask for help when lifting. Wear mechanic s gloves to avoid damaging your hands. Brake service often is dirty, and wearing gloves helps protect your hands and also helps prevent you from getting new components dirty during installation. Check all the replacement parts against the old parts. Make sure the replacement pads, rotors, calipers, hoses, and other parts are correct before installing them. Double-check all of your work by doing the following: Be sure of the torque specifications, and recheck to make sure that all fasteners are properly torqued. Ensure that all air is bled from the system if the hydraulic system has been opened. Check the operation of the brake warning light on the dash and the stop lamps at the rear of the vehicle. Always remember, the customer is counting on you to make sure that their vehicle is properly serviced and in 100 percent operating condition when you are finished. It is important to inform the customer that the brakes may feel different and that stopping performance may have changed after brake service. Pads of different lining compounds will have a different feel when the brakes are applied. In some cases, even different brake fluids can change the feel of the brakes and the feedback from the brake pedal. Be sure the customer understands that the brake pedal feel may not be exactly the same once the brakes have been replaced.

4 364 Chapter 15 Disc Brake System Inspection and Service Disc Brake Diagnosis Disc brake system problems can be grouped into a couple of specific types of concerns, as listed next. Often a problem can present itself as several of these concerns at once. A thorough understanding of the brake system is required to be able to make accurate diagnosis of brake system complaints. If necessary, review the disc brake system components and operation in Chapter 14. Common Brake System Concerns Because of the way in which disc brakes operate, they are prone to noise issues. As the rotor spins and the pads apply pressure against the rotor surface, the pads can vibrate and cause various types of squeaks and squeals. Also, disc brakes commonly experience pulsation concerns from rotor distortion. Brake Noise. One of the most common complaints is of noise from the brakes. Noise can occur when the brakes are applied or when driving without having the brakes applied. Disc brakes can make several different types of noise, with the most common being a highpitched squeal or squeak. Disc brake noise can have several causes as detailed here: Pad contact against the rotor surface. Even though a rotor may appear and feel smooth to the touch, it actually has very small peaks and valleys. As the rotor moves across the pads, these imperfections make the pad vibrate, which causes brake noise. Shims and improved pad designs are used to alter these vibrations so that any noise becomes inaudible to the passengers. Pad wear indicator as shown in Figure Many pads have this metal tab secured to the pad backing. As the pad wears to within approximately 1 /8 inch FIGURE 15-7 An example of a pad worn down to the squealer. Note the corner where it has been rubbing against the rotor. FIGURE 15-8 This pad has worn past the lining and into the backing plate. (3.1 mm), the tab will begin to contact the rotor, causing a high-pitched squeal to alert the driver to have the brakes inspected. Friction material composition. Pads with high amounts of metal often cause more noise than softer organic pads. Loose pad shims or hardware. Many pads use shims to change the frequency of any vibration so that it is inaudible, but if these shims are not installed or rust away over time, the vibration can then be heard as brake squeak. Severely worn linings. As the pad wears away, the backing can contact the rotor, as shown in Figure This causes a serious grinding sound. Disc brakes also make grinding noises if the brakes form a layer of rust on the pads and/or rotor. This occurs when a vehicle is driven in wet conditions and is then parked. The rust causes a grinding noise when the brakes are applied the first few times, but then stops as the rust wears off. A rhythmic sound like a wire brush against metal may be heard as the wheels spin and the brakes are not applied. This noise can be caused by rotor runout or thickness variation. As the rotor spins and rubs against the pads, a slight amount of noise may be generated. The noise will increase in frequency as rotor speed increases. Other causes of brake noise include contact between the rotor and pad hardware or against the splash shield. Shown in Figure 15-9, a thorough inspection of the brake assembly indicates where there is contact between components. Vehicle Pulling When Braking. The most common causes of disc brake pull are restricted hydraulic hoses and seized caliper pistons. Begin your diagnosis by verifying that the pull is related to the brake system.

5 Chapter 15 Disc Brake System Inspection and Service 365 FIGURE 15-9 The splash shield on this vehicle was contacting the rotor, creating a noise similar to a pad wear indicator. Perform an inspection of the tires and front suspension and steering components. A worn and loose control arm bushing also can cause the vehicle to pull when braking. Ensure that all suspension and steering components are in good condition. Raise the vehicle so that the tires can rotate freely, and then rotate each wheel while noting the effort needed to turn the wheel and how well it rotates after you let go. Any wheel that requires significantly more effort to turn or does not continue to rotate on its own requires a closer inspection. If all wheels turn freely, have an assistant apply the service brakes while you try to rotate each wheel again. A restricted brake hose or seized caliper piston can prevent the brake from applying, and it will continue to spin even with the brakes applied. If one wheel continues to rotate, you need to inspect the caliper and hose to determine the cause of the concern. With your helper keeping the brakes applied, open the bleeder screw of the problem caliper. If no fluid or very little fluid escapes, suspect a collapsed brake hose. If fluid does exit the bleeder, suspect a seized piston. If the hose is the cause, replace all of the rubber brake hoses. If the caliper is the cause, rebuild or replace both front calipers. Depending on the age and mileage of the vehicle, it is usually a good idea to replace the brake hoses when you are replacing the calipers. A brake staying applied after the brakes are released can also cause brake pull. This is typically caused by a restricted brake hose that allows fluid to pass when the brakes are under pressure but then blocks the fluid release. To check for this, raise the vehicle and rotate each tire. A tire that does not turn or is much harder to turn has the problem brake. To determine if the hose is keeping pressure on the caliper, open the bleeder screw. If fluid shoots out as if it is under pressure, the hose is the problem. If fluid does not come out under pressure, the caliper piston may not be retracting into the caliper when the brakes are released. If the brake is still tight with the bleeder valve open, use a C-clamp, as shown in Figure 15-10, to try to press the piston back into its bore. Even if the piston does return, the caliper should be rebuilt or replaced. End of clamp against caliper End of screw against outboard pad FIGURE If a hose or caliper problem is suspected, try forcing the piston back into the caliper with the bleeder valve open.

6 366 Chapter 15 Disc Brake System Inspection and Service Vibration. Disc brake vibration or pulsation is a common problem that is often described by the customer as a pulsating or shaking brake pedal and steering wheel shake when the brakes are applied. The vibration is often worse at higher speeds, such as on the highway. There are two main causes of this problem, brake rotor parallelism and runout. Rotor parallelism, also called rotor thickness variation or taper, refers to the thickness of the rotor at all points around the friction surface, as shown in Figure Over time, runout in the rotor, hub, or bearing creates a slight wobble in the rotor as it spins. This will cause the rotor to rub against the pads and wear away at the rotor. As the rotor wears, the friction surfaces wear unevenly. As the pads are forced against the rotor, the points where the rotor is thinner allow the pads and caliper piston to move farther out. Where the rotor is thicker, the pads and piston do not move out as far. The movement of the piston displaces brake fluid back up to the master cylinder, causing the brake pedal to pulsate. To correct parallelism problems, the rotor must either be machined on a brake lathe or replaced. Runout, either in the rotor or the hub, can also cause brake pulsation concerns. Runout is the side-to-side movement of the rotor, and it can be caused by a distorted rotor, hub flange distortion, or even worn wheel bearings. Figure shows an example of runout. Runout causes the rotor to wobble as it rotates, forcing the pads to follow the side-to-side movements. If the Exaggerated rotor parallelism 9.020" mm " mm " mm FIGURE An example of excessive rotor parallelism. Rotor Caliper FIGURE Runout is side-to-side variation from vertical in the rotor as it spins. caliper and pads are capable of floating properly on the mounting hardware, runout alone will not cause a pulsation. Figure shows how the caliper and pads can follow rotor movement to compensate for runout. Over time however, this slight bit of contact between the pads and rotor wears away at the rotor. This leads to runout induced thickness variation, which cause a pulsation concern. As the pads and hardware are exposed to the elements and rust develops, the caliper and pads may begin to stick in place. This can keep the pads and/or the rotor from being able to slide or float. When this happens, the rotor wears from the pads remaining against the rotor surface. Eventually, this runout leads to rotor thickness variations and a brake pulsation. Rotor runout can be caused by overtorquing wheel fasteners, especially by those who use air impact tools to install wheels, because the excessive torque pulls on the hub flange and through the rotor mounting face, as shown in Figure This distorts the hub and hat section of the rotor. Runout is checked by using a dial indicator and is covered in detail later in this chapter. Brake Grab. Brake grab refers to a brake that applies with too much braking force, causing the wheel to lock up easily. Also called grabbing, this condition can be caused by fluid contaminating the linings, using pads with too high a coefficient of friction, and if the hydraulic system is applying excessive pressure. A defective metering

7 Chapter 15 Disc Brake System Inspection and Service 367 Pad Pad Rotor movement forces the caliper to move back and forth with the pads Caliper valve can cause brake grab under light pressure when a person is driving on wet or slippery road surfaces. Brake Drag. Brake drag, or dragging, occurs when a brake remains applied after the brake pedal is released. This can be caused by a problem with the brake hose or by the caliper piston not retracting, as discussed earlier in this chapter. Dragging brakes can also be caused by an improperly adjusted brake pedal pushrod. Rotor Piston FIGURE As the rotor spins, the runout causes contact between the rotor and pads, which wears on the rotor. This then causes parallelism problems as the rotor wears from the contact. Abnormal Pad Wear. Brake pads should wear evenly, both the inner and outer pads equally and from side-to-side on the vehicle. Uneven pad wear can be caused by problems in the hydraulic system, such as restricted brake hoses and seized caliper pistons. Abnormal wear, as shown in Figure 15-15, is often caused by the pads no longer being able to move within the mounting hardware in which they are secured. Sticking calipers, stuck caliper pins, and rusted pad hardware prevent the pads from sliding properly. This can cause the pads to stay in contact with the rotor after the brakes are released, causing rapid and tapered pad wear, as shown in Figure Removing the rust buildup and a light FIGURE Uneven or tapered wear on the pads indicates either the caliper is not floating properly or the pads are stuck in the bracket. FIGURE Overtorquing lug nuts can distort the hub and hat of the rotor, causing runout. FIGURE Tapered pad wear can be caused by stuck pads failing to retract after the brakes are released.

8 368 Chapter 15 Disc Brake System Inspection and Service application of brake lubricant or antiseize when the pads are installed can often prevent a reoccurrence of the problem and prolong pad life. DISC BRAKE INSPECTION Begin your inspection with a conversation with the driver of the vehicle. Be sure to find out if any noises are present and if any vibration or pulsation occurs when braking. Next, start the engine and apply the brakes with the vehicle stopped. Ensure that the pedal is firm and has plenty of reserve. Note if the red BRAKE or ABS warning lamps are operational. Both should illuminate during engine start and bulb check and then go out after a few seconds. Some vehicles have electronic brake pad wear indicators that turn on a dash warning light when the pads need replaced. This light remains on after the engine is started. When it is possible, perform a test drive, listening for any noises during driving and braking, and note any pull or vibration. Calipers and Hoses. When you are performing a visual inspection of the brake system, always look for signs of fluid loss. Brake calipers can leak fluid from a faulty square seal inside the caliper. This allows the fluid to leak around the piston and out of the dust boot onto the pads and rotor. The loss of fluid reduces brake application pressure, pulls air into the hydraulic system, and severely reduces the stopping ability of the brakes. A leaking caliper must be either rebuilt or replaced. Brake hoses can also fail and leak. Inspect the hoses for cracks in the external covering layers and leaks around the fittings and clamps, as shown in Figure When a hose is leaking, all brake hoses should be replaced since all of them are likely the same age and may fail in rubbing or contact with other components, especially if steering or suspension work has recently been performed on the vehicle. A missing hose retainer or bracket bolt can allow a hose to contact the tire, which will quickly wear through the hose. Inspect Pad Wear. Pad lining thickness can often be checked with the caliper in place, but a thorough inspection requires the caliper be at least partially removed so that the pads can be removed and examined. Figure and Figure show pads that could not be inspected with the caliper in place. The crumbling apart. As these photos show, just checking the lining thickness with the pads installed does not provide this chapter. This measurement can be used to determine FIGURE This pad is cracking and crumbling. This will not be noticeable by just looking at pad thickness with the pads installed on the rotor. Remove the caliper to get a look at the pads during an inspection. FIGURE A leaking brake hose. Always check the entire brake system during a brake inspection. FIGURE This vehicle had a caliper that would not release. This overheated the pads and rotor. Copyright 201 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the ebook and/or echapter(s).

9 Chapter 15 Disc Brake System Inspection and Service 369 approximately how much life remains in the pads. In most cases the brake pads should be replaced when the lining material reaches about inches (1.5 mm). Many manufacturers have specifications for minimum lining thickness. If the lining wears below this minimum amount, the stress and heat of braking can cause the lining to crack and crumble. You should make sure the customer understands that the thinner the pad lining is, the more rapid the wear will take place. For example, a pad with half of its lining worn away may have been in service for 30,000 miles. As the pad gets thinner, the remaining half of the lining will likely not last an additional 30,000 miles. This is due to the acceleration of wear as the lining becomes thinner and is less able to dissipate heat. Inspect the pads for uneven wear. This means that both the inner and outer pads should have the same amount of lining remaining and that the lining is wearing evenly along the length of the backing material. Uneven wear, like that shown earlier in Figures and 15-16, can indicate that the pads are not sliding properly and are not fully releasing from the applied position against the rotor. Uneven inner and outer pad wear can indicate a problem with the caliper piston not retracting or the caliper itself not releasing on the mounting pins. Careful inspection of the pads allows you to accurately diagnose pad wear and correct the causes when you service the brake system. When uneven pad wear is evident, you should check all hardware carefully. Many floating calipers attach to brackets with bolts that float in the brackets. This allows the caliper to move as the brakes are applied and released. The mounting bolts are protected with rubber boots. However, in parts of the country where rust is a concern, these bolts often seize in the brackets, and prevent the caliper from floating. If they are not checked during the inspection, a seized bolt may not be apparent until you try to remove it. Too often these bolts break off in the bracket when you attempt to remove them. Inspect the bracket or steering knuckle where the pads attach. Some vehicles do not use guides, and the pads ride directly on the bracket or knuckle. This can eventually cause a wearing away of the metal, which can lead to pad noise as it rattles because it is no longer securely held. This wear can also cause the pads to bind and not to move properly along the bracket or knuckle. Disc Brake Disassembly Before you start to disassemble the brakes, first ensure that you understand how the brake is assembled. Note the locations and appearance of the bolts, clips, pins, and other parts that secure the caliper and the pads in place. You may want to take a picture or obtain a diagram or exploded view of the disc brakes from the service information in your shop before you begin to remove any parts. If provided, follow the service instructions for brake service supplied by the vehicle manufacturer. Caliper and Pad Service. On vehicles with floating or sliding calipers, begin by pressing the caliper piston back into the caliper bore, as shown in Figure End of clamp against caliper End of screw against outboard pad FIGURE Before removing the caliper, some technicians force the piston back into the bore using a C-clamp installed over the caliper. Open the bleeder when performing this task to prevent the fluid from being forced back up the system.

10 370 Chapter 15 Disc Brake System Inspection and Service Loosen the bleeder screw and attach a hose from the bleeder into a bottle to catch the brake fluid. Position a C-clamp over the caliper, and turn the clamp so that the piston is forced into the caliper. Slowly tighten the clamp to retract the piston. Going slowly reduces the stress on the square seal inside the caliper as the piston is forced back into its bore. Once the piston is back in the bore, close the bleeder screw and remove the hose. Next, unbolt the caliper from the knuckle or bracket. Use a bungee cord or wire hanger to support the caliper as shown in Figure Do not allow the caliper to hang on the brake hose as this can damage the hose. Hang the caliper from the strut or other suspension component. Do not attach the bungee cord or wire to the fender or other piece of body work. If the caliper mounts directly to the knuckle, remove and inspect the mounting bolts, sleeves, or other components that secure the caliper in place. Floating calipers must be able to move on the mounting hardware, so check that all mounting components are free. Many technicians include replacing any rubber caliper mounting bushings in the estimate when they are replacing brake pads as these bushings often become worn and damaged over time. Inspect where the pads mount to the caliper body. These locations should be cleaned of rust before the new pads are installed. If the pad contacts the caliper, like in Figure 15-22, clean these contact points thoroughly. If the inner pad rests in the steering knuckle, clean this area and apply brake lube where the pads slide against the knuckle during reassembly. FIGURE If the pads attach to the caliper as these do, clean the contact areas before installing the pads. Calipers that mount to a bracket often use a sliding sleeve or pin between the caliper and the bracket, as shown in Figure These must be free to move. Inspect the pins, pin boots, and the bore in the bracket for rust and corrosion that can prevent movement. An illustration of the pins and sleeves is shown in Figure Clean and lubricate the pins and the bracket so that the pins can move freely when they are installed. Many times these brackets use thin pieces of steel hardware between the bracket and the pad, as shown in Figure These guides are often spring-loaded to fit tightly to the bracket to prevent vibration and noise. Some replacement brake pads come with replacement guides. If they are in good condition, they can be cleaned, lubricated, and reused. However, it is a good idea to replace these hardware pieces as part of the brake service. If, they are damaged, loose, or rusted out, they must be replaced. These guides hold the pads securely to the bracket to prevent vibration and noise but also allow the pads to move as the brakes are applied and released. Many technicians replace all of the caliper hardware during brake pad service. An example of a hardware kit is Caliper mounting bolts FIGURE Do not let calipers hang by their hoses. This can stress and damage the hose. FIGURE Caliper mounting bolts and sleeves should be cleaned, inspected, and lubricated before installation.

11 Chapter 15 Disc Brake System Inspection and Service 371 Caliper housing Bushing Mounting bolt Flexible seal boot Lube exterior of bushing and bolt with brake lubricant FIGURE An illustration of a mounting bolt and sleeve for a floating caliper. FIGURE Many pad sets come with new guides. If reusing the old guides, thoroughly clean and inspect them before reinstallation. If the guides are rusted through, are not firmly attached to the bracket, or are damaged in any way, discard them and install new guides. shown in Figure This is a low-cost method of ensuring that the brakes will be in like-new condition, and it eliminates the problems of worn, rusted, or damaged parts compromising the quality of the repair. Remove the rotor to inspect it. Floating rotors usually slide off the hub with little trouble. However, rotors can rust and stick to the hub, making removal difficult. First, make sure the rotor is a floating rotor, meaning it is not held on by the wheel bearing. Also check for retaining screws holding the rotor in place. If screws are installed, remove and discard them. If necessary, apply a penetrant to the hub area and lightly tap the rotor with a plastic hammer or rubber mallet. Some rotors have two threaded holes in the hat in which bolts, typically 8 mm 1.25 bolts, can be threaded into these holes and tightened to push the rotor off the hub. FIGURE An example of a caliper hardware kit. Replacing the hardware is an inexpensive way to ensure proper brake operation and reduce the chance of a comeback. Do not hit the rotor with a steel hammer unless you are replacing the rotor. Clean the inner and outer surfaces of the hat and the hub mounting surface before you reinstall the rotor. If the rotor is trapped, meaning held in place by tapered wheel bearings, the bearings have to be disassembled to remove the rotor. This is discussed in Chapter 5. If the rotor is trapped with a sealed wheel bearing, an example of which is shown later in this chapter in Figure 15-50, the bearing and rotor are removed as an assembly. You need to follow the manufacturer s service procedures for the specific vehicle on which you are working. Removing this type of bearing often requires using a slide-hammer to separate the bearing from the steering knuckle and usually requires replacing the bearing. When you are ready to reassemble the brakes, use a small amount of brake lubricant on places where the

12 372 Chapter 15 Disc Brake System Inspection and Service Lubricate pad guides FIGURE During installation, lubricate the guides and anywhere there is movement of the pads or caliper. It is important to use the correct lubricant. Do not guess; check the service information. pads or calipers must move; an example is shown in Figure It is important that a lubricant approved for brakes is used, that the lubricant is used correctly and in the correct locations, and that excessive amounts are not used. Not just any type of lubricant can be used on brake systems. Ordinary chassis grease and similar petroleum-based lubricants should not be used in any brake system application. Disc brake lubricants are special, high- temperature greases that are either synthetic or silicone based and often contain a form of moly or graphite. The advantages of moly and graphite lubricants are that they do not migrate, meaning they stay where they are applied. The high brake temperatures do not cause the moly or graphite lubricants to thin and become mobile. Brake lubricants should be applied where there is movement between the pads or where calipers slide or move. Do not apply lubricants to the linings or backing plates between the caliper and the plate. Lastly, do not apply large amounts of lubricant on every surface. The excess lubricant will be forced out and will end up on the linings or rotor surface, which is the last place you want any type of lubricant. If the brake caliper attaches to a support bracket, ensure that the contact points between the bracket and pads are clean. Apply lubricant if it is indicated in the service information. Next, install the pads on the bracket and against the rotor surface. Make sure the pads are aligned properly and that the linings are against the rotor s friction surfaces. Carefully install the caliper over the pads, align the caliper mounting bolts with the bracket, and install the bolts. Make sure the caliper and mounting bolts are able to slide properly once they are installed. Torque the caliper bolts to specifications. Because the caliper pistons are retracted completely into the caliper bore, the brake pedal will be soft and will likely go to the floor for the first several brake applications. Before you start and move the vehicle after the pads have been replaced, pump the brake pedal several times until the pedal pumps up and is firm. Failure to pump up the brakes before moving the vehicle can result in serious injury or death, and damage to the vehicle and equipment. Never let the customer be the first person to drive the vehicle after the brakes have been serviced. Always make sure the brake pedal is firm and the brakes are working before moving the vehicle. Perform a testdrive and bed-in the pads before you return the vehicle to the customer. Pad Break-In or Bed-In. New pad break-in, bedin, burnishing, or conditioning are terms describing the process of effectively breaking in the new pads to the brake rotors. Bed-in should be performed by the technician before the vehicle is returned to the customer. However, it may require several hundred miles of normal driving to fully bed-in the pads. Performed properly, this will extend the life of the pads and reduce noise and vibration problems as an even layer of pad material is transferred to the brake rotors. If done properly, the rotors will suffer less wear, and the possibility of rotor thickness variation is reduced. How brake pads are bedded in depends on the pad manufacturer s recommendations. In general, 20 to 30 stops are made from low speed at 20 to 30 mph, with a cooling time between stops. You need to read the documentation provided with the new pads to perform the correct procedure. Improper bed-in does not create the transfer lay between the pads and rotors, or it may overheat and damage the pads. Brake Rotor Inspection During the life of the brake pads, the material of the brake rotors also wears away. Eventually, the brake rotors wear out and require replacement. When to leave the rotor alone, machine it, or replace it depends on how much wear the rotor has and the internal and external condition of the rotor. While a visual inspection of the brake rotors can provide you with information about severe scoring, external rust buildup, or even overheating, it should not be the only method of inspection. Rotors must be carefully inspected and measured to determine if they are fit to remain in service. Visual Inspection Begin your inspection by looking over the rotor friction surfaces and hat. Obvious defects, such as scoring, rust pits, and heat discoloration should be noted, as shown in Figure through Figure

13 Chapter 15 Disc Brake System Inspection and Service 373 Figure 15-28, is normal, and generally is not a concern. The materials in the brake pads and normal wear will cause shallow score marks around the rotor surface. Heavier scoring, as shown in Figure 15-29, results scoring occurs when the pad lining wears down to the rivets or backing, and results in metal-on-metal contact between the pad and rotor. Rotors that are heavily scored should be machined or replaced. Refer to the mine at what point scoring is excessive and requires service. General Motors service information states that and the rotor, if free of other defects, can be reused without machining. Rust pits, shown in Figure and Figure 15-31, are common in poor-quality brake rotors. Rust deposits form inside the cast iron, and as the friction surface wears and flakes away, pits begin to show in the surface. Machining the rotor will not fix this problem as it is a defect in the iron. Severe rusting of the rotor surfaces, shown in Figure outside, this rotor looks normal, but upon inspecting the inboard side, the extent of the rust is obvious. FIGURE Rust pitting is caused by defects in the iron and typically requires rotor replacement. FIGURE Light scoring of the rotor surface is normal. FIGURE Heavy scoring, if beyond the manufacturer s limits, will require machining or replacing the rotor. FIGURE Always check the inboard side of the rotor for defects. The outboard surface may look good, but the inboard can be in very bad condition. Copyright 201 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the ebook and/or echapter(s).

14 374 Chapter 15 Disc Brake System Inspection and Service FIGURE Severe rusting requires rotor replacement. FIGURE Overheated pads and rotors can be caused by a blocked hose or a sticking caliper piston. In severe cases, the rotor can rust in half, as shown in Figure These rotors rusted through the vents and separated the inboard and outboard halves of the rotors. Overheated brakes, shown in Figure 15-34, can cause the rotor to glaze and turn blue. This vehicle had a sticking brake caliper which kept pressure on the pads, resulting in overheating of both the pads and rotor. This allowed excessive transfer of material between the pads and rotors. Inspecting the outside surface of the rotor is easy, but inspecting the inner friction plate requires removing the caliper to get access to the rotor. There can be a significant difference between the inner and outer friction surfaces, so always check both sides of the rotor closely. To thoroughly inspect the rotor, it should be removed. With the rotor removed, inspect the hub for damage and rust buildup that can cause the rotor to sit improperly and and clean before machining or reinstalling it. AIR VENT INSPECTION Rotors do not rust only on the outside; serious rust can also occur inside vents. To inspect the inside of the vents, use a bore scope or video bore scope, as shown in Figure and Figure This allows you to look down into the vents to check how rusted the rotor is. As discussed above, rotors can completely rust away from the inside. This leads to the friction plates actually sepa- the rotor separates, it will be easy to diagnose due to the noise and pulsation. However, your customer will be FIGURE Rotors can rust in half through the vents. This usually locks the brake as the two halves separate and then wedge into the caliper and pads. FIGURE If possible, check inside the rotor vents with a bore scope. Copyright 201 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the ebook and/or echapter(s).

15 Chapter 15 Disc Brake System Inspection and Service 375 FIGURE An interior view of a rotor vent. FIGURE Rotor specifications are often stamped on the hat or on the edge of the friction surface on solid rotors. However, rust often deteriorates the surface and makes the spec unreadable. much happier if you determine that the rotor is severely rusted internally before it actually fails. ROTOR SPECIFICATIONS All brake rotors have a minimum thickness or discard specification, meaning how thick the friction plates can be or how thin the rotor can wear down to before it must be replaced. Some manufacturers will also provide a machine-to or minimum refinish spec. Examples of these dimensions are shown in Figure the condition of the rotor, you may be able to find the minimum thickness spec stamped into the hat section, as shown in Figure The reason rotors have a minimum thickness spec is because as the rotor friction plates wear away, the rotor absorbs and dissipates less stress and heat. As the friction plates wear, more heat is left to be absorbed by the brake pads. This increases pad wear and can lead to cracks in the lining material. When a rotor wears to or past the minimum or discard spec, it must be replaced. The machine-to spec is often to inches to provide a wear buffer for the rotor to remain in service. If a rotor is larger than the machine-to spec and is in otherwise good condition, it can remain in service. If however the rotor is above machine-to but requires resurfacing on thickness after refinishing must be above the machine- FIGURE An example of brake specifications. Copyright 201 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the ebook and/or echapter(s).

16 376 Chapter 15 Disc Brake System Inspection and Service of friction surface is what is worn away during normal service. Once the vehicle needs the brake pads replaced again, the rotor will likely be below the machine-to spec and will need to be replaced. Rotors also have a spec for parallelism, also called taper or thickness variation, referring to the varying thickness of the friction surface around the rotor surface. This spec is often quite small, around inches or one-half of a thousandth of an inch (0.012 mm). Slight variations in rotor thickness result in brake pedal pulsation, as the pads must move in and out to compensate for the rotor. An example of rotor taper is shown in Figure Exaggerated rotor taper Rotor runout specs, the amount of side-to-side or lateral wobble of the rotor, is often to inches (0.5 to 0.1 mm). Overtightening lug fasteners, rotor overheating, worn wheel bearings, and damage or rust buildup in the hub can cause rotor runout, which can also cause brake pedal pulsation. Rotor Measurement Rotors must be measured any time disc brake service is performed. Rotor thickness is measured with a disc brake micrometer, as shown in Figure and Figure Rotors that are smaller than machine-to or below the discard limit must be replaced. Rotors above machine-to may be able to be placed back in service depending on their parallelism, runout, and surface condition. A rotor that is above machine-to and is within parallelism and runout specs and has no surface defects can often be reused without machining. Many manufacturers state that rotors that meet all specs and have acceptable surface conditions should not be machined and should be reinstalled as they are. This is because machining removes material, and makes the rotor less able to absorb and dissipate heat. Also, improper machining methods FIGURE An illustration of rotor taper. FIGURE An example of a metric rotor micrometer. This mic reads to 0.02 mm. FIGURE An English rotor micrometer. This mic can read to inches.

17 Chapter 15 Disc Brake System Inspection and Service 377 FIGURE The markings and graduations on an English rotor mic. Measure 8 12 points X X X FIGURE The markings and graduations on a metric rotor mic. can cause increased braking noise and customer dissatisfaction. Measuring rotor parallelism requires a micrometer that can measure at least to inches or about 0.01 mm. Many brake micrometers use a graduation that indicates half-thousandths, like the mic shown in Figure This graduation allows you to determine if the rotor is parallel to within inches, which is a common spec, although some manufacturers specify parallelism to be less than inches. An example of the graduations on a metric mic is shown in Figure Measure the rotor at eight to twelve places around the friction surface as shown in Figure The reason so many measurements are made is that you are measuring approximately the size of the lining contact area between each point. X Micrometer FIGURE Measure rotor parallelism at eight to twelve points on the rotor. A rotor with excessive parallelism will need to be either machined or replaced, depending on its overall thickness. Rotor runout is measured with a dial indicator, as shown in Figure If the reading is beyond the spec, remove the rotor and reindex it on the hub 180 degrees (if possible) from its original position, then remeasure the runout. This will help you determine if the runout is with the rotor or the hub. Rotor runout can also be caused by excessive rust buildup between the inside of the hat and the hub. If runout is excessive, remove the rotor and inspect the inner surface of the hat and the hub. Remove rust buildup with a wire brush, reinstall the rotor, and

18 378 Chapter 15 Disc Brake System Inspection and Service because that can create the runout problem. If the hub runout is acceptable and the runout is in the rotor, on-car machining of the rotor may solve the runout. If the rotor is below thickness specs, you need to replace the rotor and remeasure the runout. If it is necessary, reindex the new rotor until the smallest amount of runout is obtained. FIGURE Using a dial indicator to measure rotor runout. remeasure the runout. When the brake service is complete, the rotor should be installed so that the minimum runout is present. If the reading is still excessive, remove the rotor and measure the hub runout, as shown in Figure If the hub runout is excessive, the hub may need to be replaced. Ensure that the wheel bearing is not worn and loose Rotor Refinishing and Brake Lathes Once a common practice, performed every time the brake pads were replaced, brake rotor machining is now done less frequently due to the smaller and lighter rotors have enough material to last until the brake pads need to be replaced but not much more. Because of this, it is very common to replace the brake rotors and the brake pads as the rotors are too thin to be machined and returned to service. WHEN TO REFINISH A ROTOR As stated previously, some manufacturers say to reinstall a used brake rotor without refinishing it if it is within wear specs and does not have any major surface condition problems. However, there are times when you need to refinish a rotor to correct a problem. OFF-CAR BRAKE LATHES Off-car brake lathes, like the one shown in Figure 15-47, have been in use for many years in the auto industry. These lathes are typically used on both brake rotors and brake drums. Before you attempt to use a brake lathe, some basic safety must be observed. Dial indicator FIGURE Excessive rotor runout can be caused by runout in the hub or a loose wheel bearing. Do not replace the rotor until the true cause of the runout is found. FIGURE An example of a common off-car brake lathe. Copyright 201 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the ebook and/or echapter(s).

19 Chapter 15 Disc Brake System Inspection and Service 379 Brake Lathe Safety. Never use a brake lathe without wearing safety glasses or goggles. Also, all guards and shields must be in place before use. The brake lathe shaves material off the rotor, and throws off small, hot, and sharp metal shavings that, if they get into your eyes, can cause serious eye damage and loss of sight. The lathe uses a rotating spindle on which the rotor is mounted. Be sure that you do not have any loose clothing, jewelry, or hair that can be pulled into the rotating parts. The motor on the lathe has a lot of torque and can pull you into the machine if you become entangled in it. Setting the Lathe to Machine a Rotor. There are different types of rotors solid, vented, composite, hubless and each requires specific mounting and machining procedures. Refer to the documentation that comes with the lathe you are using before you attempt to mount a rotor and make any cuts. The steps shown in Photo Sequence 9 are typical for mounting a cast iron hubless or floating rotor. Once it is mounted, follow these steps: Center the cutting head over the rotor. Place the cutting bits about halfway across the rotor surface, and turn the lathe power on. Adjust the cutting bits until each just touches the rotor and makes a scratch cut. Back the bits off the rotor and turn the lathe off. Loosen the rotor mounting, turn the rotor 180 degrees on the arbor, and retighten. Perform a second scratch cut. The two scratch cuts should be parallel; if not, remove and inspect the mounting components, clean and remount as needed. Set the cutting bits halfway across the rotor and turn them until they just touch the surface. Set the depthof-cut collars to zero. Turn the lathe on and move the cutting head in toward the hat. Set the depth-of-cut between inches and inches (0.15 mm 0.25 mm). Engage the feed for a fast or rough cut. If the rough cut cleans up the surface and removes the low sections, reset the lathe to perform the slow or finish cut. This cut is usually a more shallow cut, between inches and inches (0.10 mm mm). Repeat the cutting process for the final cut. Once complete, apply a nondirectional finish to the rotor. Remeasure rotor thickness and clean thoroughly. Rotors are usually rough cut, and then a finish cut is performed. The rough cut removes the surface defects and restores the surfaces to parallel. If the surface is heavily scored, a second rough cut may be necessary. Once the surface is cleaned of defects, a finish cut is FIGURE A finished rotor with a nondirectional surface finish. performed. This cut is much slower and cuts less material than the rough cut. Once the finish cut is complete, a nondirectional surface finish is applied using a light abrasive. This breaks up the lines that appear in the rotor surface from the cutting bit. Figure shows a rotor after the nondirectional finish is applied. Breaking up the lines in the rotor surface helps reduce noise produced by the pads and rotor during braking. Once the rotor is finished, recheck its finished thickness with a micrometer to ensure that it can be put back into service. If the rotor is usable, it must be cleaned before being reinstalled on the vehicle. Even though the rotor appears clean and like new, the surface has a very fine layer of metal dust embedded in the valleys of the metal. Wash the rotor with a warm soapy water solution to remove the dust, and dry completely. On-Car Brake Lathes On-car brake lathes perform the same function as off-car lathes except that the on-car lathe attaches to the vehicle in order to machine the rotor. Why Use On-Car Lathes? Some vehicle manufacturers require the use of an on-car lathe to machine the brake rotors. The main reason is to eliminate runout problems. Figure shows an example of what is called stacked tolerances, meaning that several components, all with a spec for allowable runout, are mounted together, thereby adding the runout of each component until the total runout is excessive. Removing the rotor and machining it with an off-car lathe does not remove the runout present at the rotor. By using an on-car lathe, the lathe compensates for the runout present at the rotor, making the finished rotor true to the vehicle, eliminating rotor runout-induced problems. A second reason for using an on-car lathe is that improper setup of a rotor on an off-car lathe can actually

20 380 Chapter 15 Disc Brake System Inspection and Service PHOTO SEQUENCE 9 SETTING UP A ROTOR ON A BRAKE LATHE PS9-1 Determine the correct adapters required to mount the rotor to the brake lathe. PS9-2 Install the open cone onto the spindle followed by the spring and the hub adapter. PS9-3 Position the rotor onto the spindle with the hat outward, just as the rotor is installed on the vehicle. PS9-4 Install a second open cone against the outside of the hat. PS9-5 Install a spacer as needed to take up space on the spindle out to the threads. PS9-6 Install the spindle nut. Note that the threads are left-handed. PS9-7 Install the vibration dampening strap around the rotor vents and secure it in place. PS9-8 Turn the lathe on and check for excessive rotor movement. If the rotor moves excessively from side-to-side or up and down, turn off the lathe and remove the rotor. Check to see if the rotor is sitting properly on the hub adapter. Copyright 201 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the ebook and/or echapter(s).

21 Chapter 15 Disc Brake System Inspection and Service 381 Drive shaft The wheel bearing is allowed a slight amount of play. Wheel bearing assembly Wheel hub The hub is allowed a slight amount of runout. Lock washer Steering knuckle Cotter pin Brake rotor The brake rotor is allowed a slight amount of runout. Wheel bearing locknut Adjusting cap All of these can add up to an excessive amount of rotor runout even though the rotor itself does not have significant runout. FIGURE Excessive rotor runout can be caused by stacked tolerances, where each component is allowed a slight amount of runout, but the sum of the tolerances results in excessive rotor runout. make rotor runout worse. Using the incorrect adapters and/or not having clean mounting surfaces on the rotor can cause runout on the lathe arbor. Machining will not correct any technician-induced runout, and the rotor, when it is reinstalled on the vehicle, will have as much or even more runout than when it was removed. Also, machining a rotor with runout in the hub with an off-the-car lathe does not remove the runout in the rotor. On-car lathes are also used because on some vehicles, the rotor is trapped or captured, meaning that it mounts inboard of the hub and bearing, as shown in Figure and Figure To remove the rotor, the bearing must be removed. This is time consuming and damaging to the bearing. Instead of removing the rotor, an on-car lathe is used. This eliminates runout issues and saves the technician time. FIGURE An example of a trapped rotor. To remove the rotor on this vehicle, the hub, bearing, and rotor assembly is removed. This was common in many Japanese-built cars. FIGURE Another example of a trapped rotor on a latemodel Chevrolet truck.

22 382 Chapter 15 Disc Brake System Inspection and Service The disadvantages to on-car lathes are that they are more expensive than off-car lathes, and if not self-compensating for runout, they require additional training, time, and skill to use. On-Car Lathe Use. Begin by removing the caliper and supporting the caliper with a wire or bungee cord, so it does not hang from the brake hose. Next, position the lathe at the hub. Select the correct adapter that matches the lug pattern and tighten the adapter using the vehicle s lug nuts. A lathe installed on a rotor is shown in Figure Depending on the model of lathe, runout compensation may be automatic or manual. You will need to read and follow the lathe manufacturer s instruction manual to compensate for runout. In general, on a lathe with automatic compensation, turn the lathe on, and press the Compensate button. The lathe then detects and adjusts for any runout. On a manually compensating lathe, you need to make a series of adjustments to the runout dials until the lathe is matched to the hub and rotor. Once the lathe is set, the rotor is machined just like when using an off-car lathe, as shown in Figure FIGURE An example of an on-car brake lathe installed on a rotor. After the finish cut is complete, remeasure the rotor thickness, and compare it to specs. If the rotor is above spec, clean the rotor with a soapy water solution to remove all metal dust and dry thoroughly. Integral Parking Brake Caliper Disc Brake Service Rear disc brake service is performed just like front disc brake service, except in the case of retracting the caliper piston on integral parking brake designs. Since these pistons are used by the parking brake system, there is a mechanical method of applying the piston in addition to the hydraulic pressure of the service brakes. To retract the caliper piston, install the tool shown in Figure 15-54, and rotate the piston back into the caliper bore, as shown in Figure The tool pushes the piston back into the bore as the piston rotates, so it is important to keep the tool secure in the caliper body during use. Once the piston is fully retracted, remove the tool from the caliper. Install the caliper and pads over the brake rotor, and test-fit the pads. There may be a slight gap between the pads and the rotor, especially if the rotor is not replaced. If the gap is excessive, use the piston tool to rotate the piston one-quarter to one-half of a turn, depending on the number of pad-locating notches the piston has. Your goal is to have the pads as close to the rotor as possible without causing drag on the rotor. If turning the piston causes the pads to be too far out, to the point where the pads and rotor will not fit over the rotor, return the caliper piston to its fully retracted position. Once the pads are installed, install the caliper and torque all fasteners to specs. Apply the brakes several times and apply the parking brake. Make sure the parking brake applies fully and locks the wheel. In some cases, the gap between the pads and rotor will need to be adjusted before the brake applies fully. This may require applying the parking brake many times to adjust the piston out to take up the gap. FIGURE This rotor is being machined on the car. Note the low spots in the rotor as the cut has moved across the surface. FIGURE A caliper piston retraction tool.