Hydraulic Brakes: How and Why

Transcription

1 Hydraulic Brakes: How and Why Hydraulic disc brakes have been fitted to Moto Guzzi for over forty years, and yet a surprisingly large number of people don't properly understand how they work. The disc brake system is relatively simple and efficient. The disc is attached to the motorcycle wheel, and has a caliper which grips onto the disc to brake the wheel. As most of the disc is running in the open air the brake is cooled efficiently - an important factor as increased heat makes it harder for the caliper to grip the disc. The caliper grips the disc by squeezing two "pads" of friction material onto the disc, one either side. The pads have to be squeezed very hard - so hydraulics are usually used to give the necessary leverage to apply sufficient pressure. When the handlebar lever/or foot lever is operated, fluid is forced out of the master cylinder and into the pistons in the caliper via a hose. This pushes the pad onto the disc. Most of us know this much, but many do not know how the hydraulic components work. All hydraulic master cylinders operate in the same way. The piston (see diagram) is pushed down the length of the housing. The housing is kept permanently full of hydraulic fluid, and the piston traps a quantity of fluid in front of it. The fluid is pumped through the check valve and into the hydraulic line, exerting pressure on the piston at the other end. The harder you push the piston in the master cylinder, the greater pressure it exerts on the brake pads.

2 When the brake is released the fluid moves back into the master cylinder body. This has to come back via the check valve. When you squeeze the brake on, fluid travels through ducts in the valve, but when the fluid is coming back it forces a seal home, covering the ducts. Thus it has to force its way around the check valve. The piston, however, is forced back by the spring quite quickly, so before the fluid can all return a small amount of fluid is sucked in front of the piston from the fluid behind it. The main seal in the piston acts like a one-way valve, collapsing and allowing the fluid past it. The piston has ducts drilled in it to allow the fluid through. This washes and lubricates the seal. When the piston has returned fully, the fluid returning from the rest of the system returns to the reservoir through the now uncovered relief port. This relief port also allows the fluid to expand/contract without affecting the system. The cylinder behind the main piston seal is kept filled by the main port, and the hydraulic oil held in by an end seal. There is a rubber boot keeping water and dirt out at the end of the cylinder. This normally has traces of hydraulic oil in it that has wept past the end seal- but this acts as lubrication and protection and does no harm to the system. As long as the reserve is kept topped up the pumping of the piston replaces any oil lost from the system - so it's self-adjusting as well. To avoid confusion it is worth mentioning that on the V5011 the oil reservoir is mounted separately from the master cylinders and connected to them by piping. Otherwise they act exactly as the conventional set up. The V5011 shows another uncommon feature, in that the master cylinder for the front brake is operated via a cable, and not directly on the handle bar as most other bikes. At the other end of the system we have the brake caliper. On Guzzis they are of the twin-acting caliper type. This means that the calipers house 2 pistons, one pushing from each side. (See diagram). When the master cylinder is operated the fluid is pushed into the main caliper casting and pushes out against the pistons, these in turn squeezing the pads onto the disc. The caliper body is sealed to the piston by a main, square section seal (not to be confused with the outer seal against water/dirt etc.). This is placed in a groove machined inside the caliper, the base is machined at such an angle so the seal is deformed, and presents an edge against the fluid.

3 When the piston is pushed out the seal distorts, and when the pressure is released the seal pulls itself back into its free shape, pulling the piston back off the pads, thus allowing the pads to come clear of the discs (to about 7 thou - normal running clearance). As the pads wear the piston slips out further past the seal, and sits with more fluid behind it. Thus as long as the pads are OK and the fluids topped up, these too are self- adjusting. One thing which is almost unique to Moto Guzzi is the linked brake - or the integral brake system. As we all know, the linked brake is operated by the foot pedal and activates the rear caliper and one of the front ones. The idea of this is to achieve the optimum balance of braking forces without relying on rider skill. This system was developed by Moto Guzzi and Brembo, and first used on a production bike in The first integral brakes shared the pressures in the hydraulics by having different caliper piston sizes in the front and rear brakes. This was later superceded by the present system, i.e. by linking the two calipers by a special valve, thus giving 75% of braking pressure to the front and 25% to the rear. This system gives a near perfect balance of brakes for all but emergency braking from high speeds, or hard braking when the bike is heavily loaded. In either of these cases the front handlebar operated disc can be used to compensate. All in all, Moto Guzzis come equipped with what many people feel is the best braking system available for a road bike. Keeping your brakes in good working order can present some problems, but if checked regularly there is no reason why your brakes shouldn't be as good as

4 new. Firstly you should always check your brake pads - if they are worn replace them. To keep your brakes working properly they should be kept clean. Dirt or corrosion on the pad locating pins will hinder the working of the brake, as will any oil/grease/dirt etc. on the disc. Part of routine servicing of the brakes is "bleeding" the hydraulics. For the hydraulics to work properly they must be full of clean hydraulic oil. If air or water contaminates the system (as it always does slowly) the old oil must be bled from the system and replaced by new. Any air in the brake calipers will collect at the highest point, i.e. directly underneath the bleed nipple. Bleeding this off should be simple. Air in the hydraulic lines will find its way to the highest points; the master cylinder on the handlebar brake, or on the integral brake air in the system can collect in the distributor valve. This can take an awful lot of getting out unless your bike has a bleed nipple on the valve. If it doesn't you have to chase the air with fluid all the way to the calipers to get it out - this can be a long, irritating job. If in doubt I find using a brake fluid of a different colour is useful. Then you keep on bleeding until you have replaced all the oil with the new oil of a different colour. It is probably worth mentioning that the popular rumour that you can't mix different makes of brake fluid is rubbish. You can mix any make of fluid that comes in a can marked "suitable for disc brakes", and carries the same Dot rating. Dot 4 etc. A not uncommon problem is bleed nipples shearing off in the caliper. As bleed nipples have to have a hole through them they are not as strong as they might look. Over tightening of the nipples is the commonest cause, though if a nipple is corroded in place they often shear off before they unscrew. If your bleed nipples are old or in any way suspect, then replace them. They re cheap. Hydraulic oil is funny stuff. When new it is virtually incompressible, but it absorbs water with a vengeance. (The rubber seal in the master cylinder reservoir is there to separate the oil from the air which contains water vapour). If it absorbs too much water it is no longer incompressible, so has to be replaced. It follows that any oil that has been in contact with the air has absorbed water vapour, so as the makers say, only use oil from a sealed container! If your bike is getting on a bit (as most are) the whole braking system should be inspected for damage. Any lengths of hydraulic line that are damaged should be replaced. The old motto - "If in doubt, chuck it out", certainly applies here. When you lean on your brakes at 110kph because some idiot in a truck pulls out on you is no time to find you've a cracked line! When you do replace a brake line the system has to be extensively bled to chase out air - I have found that it is impossible to get the brakes perfect in one go. However much you bleed it some air is hidden. Once the brakes work O.K., but still feel a bit spongy, leaving the bike for a week

5 or so is in order - then bleeding the remaining air out is easy. This allows the small pockets of air to rise to the highest part of the system so it can be chased through in one go. If you find that despite regular maintenance your brakes aren't up to scratch it is likely that the source of the problem is one or another of those all important seals. Firstly check all the banjo bolts and unions for leaks - where oil gets out air can get in - if these are all O.K. an actual oil seal is probably worn out. If it is a caliper seal it will be the inner square section seal allowing oil to leak around the caliper piston, thus losing pressure that should be applied to the brake pad. If this is the case the leaking oil will probably be visible, and quite often gets onto the disc to create more problems. If the caliper seal is O.K. then it will almost certainly be the main seal in the master cylinder. If this seal is worn out then oil leaks around it in the master cylinder, thus losing brake pressure. As the oil merely travels past the piston into the area not under pressure the oil will probably be kept in the system by the end seal, so there will be no external evidence of anything amiss. When replacing the seals the caliper/master cylinder should be carefully cleaned and dried. Use metho for this. On no account use a cleaning agent containing water, as this will contaminate the system, or anything mineral based. Petrol, kero or any mineral based oil will be absorbed by the oil seal causing it to swell and seize. New seal kits can be bought mail order and quite cheaply from most of the suppliers. If you replace the master cylinder main seal then buy the whole kit and replace the return spring. This spring not only returns the piston but also keeps the check valve in position. If this spring is "tired" then the check valve will not function properly and the piston main seal will not be "washed" as the piston returns. This means it would soon cease to work efficiently. If despite all this your brakes don't work, well there are Moto Guzzi dealers. MERLYN