Braking Systems Part 3
by David Grainger
President, The Guild of Automotive Restorers
In this installment on doing your own brake work we'll cover the honing of your wheel cylinders and the bleeding of the system.
If your brake cylinders had minor pitting or a slightly rough feel or small inner ridge they are candidates for honing. Many professionals give a cylinder a minor honing if it feels rough or not and this is generally a good idea as it gives the bore a fresh surface to seal. A brake hone is a small attachment that installs on your electric drill. It is composed of a center stock from which there are three spring-loaded arms, at the tips of which are three small whetstones.
This is inserted into the cylinder bore and passed up and down slowly a few times until the bore is fresh. The honing process needs to be lubricated and to do this you dip the stones into and wash the bore with fresh brake fluid. Use only brake fluid because it will not contaminate the cylinder like honing oils will do.
Cylinder honing brakes is not a heavy duty job. You are not preparing the bore for re-sleeving. What you are doing is removing light corrosion and preparing the bore by giving it a fresh surface. If roughness or pitting persists after a few passes of the hone then it may mean that you will need to replace or have the cylinder re sleeved.
Using a moderate speed setting on your drill, pass the stones up and down the bore until you see the tips appear at either end. I should stress that this is not a high speed process so you will need to use a variable speed drill and go about half way with the speed. You'll get the feel for at as you do it.
Once you have finished the honing you should flush out the cylinders thoroughly with fresh clean brake fluid, making sure not to miss the two ports on the bore, one for the brake line and the other for the bleeder. If the bore is not cleaned out well the small holes for these two can stay blocked with a paste composed of brake fluid and metal filings. Once you have flushed the cylinder wipe it thoroughly and give it a close inspection before re assembling it. Make sure that it is not only clean but that the bore is smooth and has a uniform appearance when you look down it. Any remaining roughness, ridges or discoloration remaining in the bore will mean additional honing or replacement.
If you have the slightest doubt about the interior bores of your cylinders being ready for re assembly, throw the cylinders in a box and head down to your local garage. Have a professional give his opinion about the finish of your cylinders. It is always better to be safe than sorry and brakes are nothing to mess around with.
Once the cylinders are honed properly you can re assemble the seals, springs and plungers that inside.
Now, another major warning. Make very sure that you know which order everything goes in, and in which direction it faces. If you put something in the wrong place or back to front, the brakes won't work. When you are dismantling the system make notes or sketches about the order and direction of the interior components. This is critical for any of the parts but the master cylinder can sometimes be fairly complicated inside in the order in which the plungers, valves and springs are arranged. Don't take a chance on your memory or on guess work.
Never put in used seals either. You may have to re-use springs and plungers but always replace the rubber cups and before you put them in inspect the edges of the cup carefully for irregularities or small scratches. Even a small scratch in the surface of the rubber skirt on the cup can cause leaks. If the brake re build kit that you got is an old one you should also check the cups for elasticity. Old rubber tends to dry and harden which means that it likely won't seal properly or it will wear very quickly.
If the cups aren't soft, especially around the edges then you should obtain newer cups. Fear not, Brake cups are very widely available at automotive jobbers and the size that you need will be embossed right on the rubber in the center of the cup. For instance you should see a marking like 1 ¼ or 1 1/8 written clearly along with the manufacturers name. Pass that information on to your jobber and they should be able to fix you up.
One last job while you have the cylinders off the car. Make sure that the bleeder screws are loose and clear of debris. If possible replace them with new bleeders also available at your jobber. If the bleeders are seized, plugged or if the flats surfaces on them are rounded off you will not be able to bleed your brake system to get it operating so make sure that you check them off the car rather than when you are lying on your back underneath it.
Once your cylinders are finished, re install them on the car and re assemble your brake system then you will be ready to bleed the system. Brake bleeding requires two people but is even better with three. One person is at the wheel, loosening and re tightening the bleeder, one is in the car pumping the peddle and if you have a third, they are monitoring the amount of fluid in the master so that it doesn't get too low and re introduce air into the system. Annoying as it is the air that you are striving to remove. If you don't have a third, one of you will have to check the master after a few pumps and refill as required.
Bleeding the brakes is a rather irksome task which should be easy and quick but can sometimes take a long time and drive you nuts.
You start the process by filling the master cylinder then starting at the wheel furthest from the master, open the bleeder then close it to make sure it is operating smoothly. With the bleeder closed, have your helper pump the brake pedal a couple of times and then hold it down. You then open the bleeder which will allow brake fluid and air out. Make sure that your helper keeps the pedal down until you re close the bleeder. If they don't then they will suck air back into the system through the bleeder screw. So as to avoid getting brake fluid all over the bottom of the car you can push a piece of clear plastic tube over the end of the bleeder nipple. It will need to be clear so that you can see the air bubbles coming out. Remember that brake fluid attacks paint so take the appropriate steps.
When the fluid clears of air bubbles and is running clear and clean close the bleeder and proceed to the next wheel which should be the opposite rear. After bleeding wheel you should have a good firm pedal which does not need any pumping to bring it up. If your brakes are power then you should with the car running have a firm but not hard pedal. If your pedal is soft and needs to be pumped to come to the top you still have air in the system or perhaps a leak somewhere. If after repeated bleedings you still can't get the pedal to stay firm then you have a leak in the system somewhere and you will have to go over to ascertain where. If you have power brakes and the pedal feels rock hard then you have a problem with your vacuum booster which can either be a rod adjustment between the master and the vacuum booster or a diaphragm problem. If you suspect a vacuum booster problem check your manual or take it to a professional.
Be patient when you are bleeding the brakes. You may have to go around the car a couple of times before they are bled properly but don't get fed up and call a "Good enough". With brakes they have to be perfect. Remember that if something happens you are responsible both morally and legally, so do it right and if you have any problems that persist, have the system and your work checked by a licensed mechanic.