The Braking System Part 2
by David C. Grainger
President, The Guild of Automotive Restorers
Hydraulic brakes rely on the building of pressure within a closed system which expands rubber and metal plungers within the wheel cylinders which then push the brake shoes apart.
These cylinders are usually made from cast steel or iron and this is where most of your braking systems problems begin if the car is used infrequently or has been in storage for any length of time. All of the procedures and problems outlined for the wheel cylinders also apply to the master cylinder which is the unit that creates the pressure and transmits it to the wheel cylinders.
What many of us don't realize is that brake fluid absorbs moisture. In day to day operation this is not really a problem because the constant agitation keeps the trace amounts of water in suspension. When a vehicle sits for any length of time the water comes out of suspension and because it is heavier than the hydraulic fluid it sits at the bottom of the cylinders as well as brake lines. This causes corrosion over time and because the cylinders rely on an extremely smooth bore in order to develop the pressure required to operate the brakes, this rust poses a problem. What occurs in the rusted areas is that the rough surface caused by the corrosion ruins the smooth fit of the rubber plugs that seal the fluid in the cylinder by roughing up the edges of the cups. This causes leaks which either reduces or totally eliminates the systems ability to develop pressure. No pressure, no brakes. This problem can manifest as a slow leak which can cause brake shoe contamination and empty your master cylinder reservoir of fluid, or can cause a catastrophic failure where the brake pedal slams to the floor just when you are trying to stop quickly. In either case the car is unsafe. Any braking systems are suspected of having problems such as leaks or corrosion caused by storage or wear and tear absolutely sideline the car until they are fixed.
So, how do you fix them? Once you have removed the brake shoe assemblies which we covered in the last column, you should have easy access to the wheel cylinder. At this point you can do one of two things. You can inspect them without removal; from the backing plate or remove them and take them to the bench. If they are in need of work, they should be removed from the backing plate in any case but first I'll briefly describe how to inspect them for damage while still on the car.
Caution: Many amateur, and unfortunately, professional mechanics who should know better, often use clamps or vice grips to pinch the rubber flex hose that feeds the cylinder its brake fluid. This isolates the cylinder from the rest of the system and makes bleeding the brakes easier and quicker later on. I will not mince words here. It is a really stupid thing to do, no matter how much easier it makes bleeding the system later.
You can get away with it with a brand new flex hose, at least most of the time, which has been recently manufactured and installed because the rubber is fresh and supple. If it is older, or a new old stock part (I have also seen brand new do this) the pinching of the flex hose can cause damage to the inside of the hose and although the hose looks like it has recovered from the pinch, a partial or even a complete restriction can occur inside the hose because what you are looking at on the outside is just a sheath. Flex hoses are composed of layers and the inner most tube can collapse and cause you grief. Partial restrictions can cause severe pulling when coming to an emergency stop and complete restrictions of course render that brake inoperative. So you say, a complete restriction would be detected when you bleed the brakes wouldn't it? Yes it would, but pinching can lead to a failure of the inner liner well after the actual damage has been done so you can bleed your brakes and think everything is just fine only to have problems later.
The first thing that you will do to examine the cylinder in place on the backing plate is to remove the end dust caps. These are rubber caps that cover each end of the cylinder and stop the admission of brake dust and other contaminants from accumulating around the ends of the cylinders. They pop off easily and there should be no brake fluid in them. If they are saturated with brake fluid, go no further, you have to take the cylinder off the car, it is already too far gone to use.
Once the caps are off, you should be able to work the plungers and brake cups out of the bore. Nothing in here is under pressure but there is usually a spring in the between the two ends so once you work one end out be prepared for that spring to want to propel the bits and pieces to areas under the car where they are hard to reach. If you can't draw them out, pushing in on the opposite end should pop them out. If they won't budge, don't worry about damaging them. They are already messed up and the cylinder will need to go to the bench.
Once you have the brake cups out, take a flashlight and get a good look inside the bore, looking for any rust or corrosion. If the fluid was dirty and you can't tell, use some fresh brake fluid on a clean rag and wipe out the bore until you can get a good look at clean metal. If it appears smooth and un-corroded then run your finger up inside the bore. If you feel the slightest roughness or any detectable ridges, the cylinder is no good and will need to be honed or replaced. Even a little roughness or the slightest ridge is too much. To be useable that bore must be perfectly smooth.
Now, here is a hard learned lesson that I will pass on. Not all perfectly smooth bores are good. Two things can be wrong:
First, if the cylinder was leaking but the bore is all right, it could be that the cylinder has been serviced once too many times and the last honing made the bore too big to allow the correct brake cups to contain the fluid. If this is the case, you can't just go oversize, although I have seen it done by those who may be a few bricks short of a load. You have to either replace the cylinder or have it re-sleeved to original spec. If I sound a little abusive towards those who gimmick up brakes, you could be right. If a braking system fails on a '58 Caddy and it slams into the back of a tiny little Geo or just about any other modern car, chances are it will not be stopped until it has passed just about all the way through the passenger compartment of the car ahead or swatted it out of the way like a fly. This is not something you take a chance with of not happening. If it happens it is not only an unfortunate tragedy, it is a criminal matter, and if investigators determine that the fault was caused by incorrect or dangerous repair procedures, they will come looking for the person responsible. This is actually one good reason to make sure that any repairs that you do to your brake system are checked by a professional. No one can stop you from working on your own brakes, but if an accident occurs because of the work that you have done, you will be held responsible. Brakes are really not like carb swapping or dropping a new set of headers on a car.
Second, and this can be a difficult one to pin point because it is rare, but does occur, is the cylinder being a little bit ovaled. If the inner bore has worn so that it has become slightly oval, you will not be able to get it to seal properly. It will need to be re-sleeved or thrown out because you can't hone it back to the correct dimensions. Cylinders going oval can be caused by brake fires, overheating or just plain wear, and the older the cylinder, the more common this fairly rare problem becomes.
If you have determined that the cylinder is damaged by corrosion or wear and you have to take it off the car, it will usually be held on to the backing plate by two bolts and of course the brake line. In part 3 of the Braking System I'll describe what gets done once you have the cylinders off the car and onto the bench.