Could Be Just Air.
A.R. : I swapped out the front drums in my 1968 Malibu for disks, replacing the master cylinder and proportioning valve. After a few days of driving I began to notice every once in a while, when sitting a stop light or at very low speeds, it seemed like the brake pedal would lose a little pressure. I asked a few people who said it was probably a leak somewhere. Can anyone suggest how to go about finding this leak? I had to replace a few lines in the front end, but I cannot find a serious leak in them. I have not taken apart the rear drums yet, but is it possible the wheel cylinders are leaking?
G._. : It sounds like you have air in the line. Bleed the brakes again. Start with the front and make sure that you also bleed the proportioning valve and the master cylinder.
S,_. : Does the fluid level in the master cylinder go down? If yes, you have an external leak, and the rear wheel cylinders are a great place to start looking. If you do not have to add fluid, then you have an internal leak, and the master cylinder is defective. A brake pedal will not sink to the floor, sometimes, unless there is a leak. Trapped air will cause a low pedal every time. Give your master cylinder the 60 second test: Get in the car, engine running if you have power brakes, and dynamite the brake pedal as hard as you can. Hold the pressure for one minute. Your leg will be sore for half an hour. If the pedal sinks, look for fluid leakage. If the pedal does not sink, then release the pressure and apply "normal" pressure - hold it for 60 seconds. Does the pedal sink? Look for fluid leakage. If the pedal sinks either time and you cannot find fluid leakage after checking the entire hydraulic system, then replace the master cylinder.
B.T. : I agree with G_. It sounds like the master cylinder to me. Sometimes they leak internally and the fluid may or may not come out. The master cylinder may leak into the vacuum booster so you will not notice the fluid loss. Bleed it first.
G.M. : The symptoms indicate a bad master cylinder. Hopefully it just needs a good bleeding. You can get lines that hook up where your brake lines hook up so when you pump the brake pedal it will shoot the fluid back into the reservoirs. You can also have someone pump up the brakes and hold them as you would for normal bleeding, and then you loosen the line fitting at the master cylinder, allowing any air to escape. There will be fluid too, so cover any painted surface you do not want ruined by brake fluid. Then retighten it before releasing the brake pedal. I believe you should do the front line first, and it will probably take a few bleedings to get all the air out.
Archivist: Tom Wilson
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