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Discussion Starter #1 (Edited by Moderator)

  • Could Be Just Air.

    A.R. : &nbsp I swapped out the front drums in my 1968 Malibu for disks, replacingthe master cylinder and proportioning valve. After a few days of driving Ibegan to notice every once in a while, when sitting a stop light or at verylow speeds, it seemed like the brake pedal would lose a little pressure. Iasked a few people who said it was probably a leak somewhere. Can anyonesuggest how to go about finding this leak? I had to replace a few lines inthe front end, but I cannot find a serious leak in them. I have not takenapart the rear drums yet, but is it possible the wheel cylinders areleaking?

    G._. : &nbsp It sounds like you have air in the line. Bleed the brakes again. Startwith the front and make sure that you also bleed the proportioning valveand the master cylinder.

    S,_. : &nbsp Does the fluid level in the master cylinder go down? If yes, you havean external leak, and the rear wheel cylinders are a great place to startlooking. 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 thefloor, sometimes, unless there is a leak. Trapped air will cause a lowpedal every time. Give your master cylinder the 60 second test: Get in thecar, engine running if you have power brakes, and dynamite the brake pedalas hard as you can. Hold the pressure for one minute. Your leg will be sorefor half an hour. If the pedal sinks, look for fluid leakage. If the pedaldoes not sink, then release the pressure and apply "normal" pressure - holdit for 60 seconds. Does the pedal sink? Look for fluid leakage. If thepedal sinks either time and you cannot find fluid leakage after checkingthe entire hydraulic system, then replace the master cylinder.

    B.T. : &nbsp I agree with G_. It sounds like the master cylinder to me. Sometimesthey leak internally and the fluid may or may not come out. The mastercylinder may leak into the vacuum booster so you will not notice the fluidloss. Bleed it first.

    G.M. : &nbsp The symptoms indicate a bad master cylinder. Hopefully it just needs agood bleeding. You can get lines that hook up where your brake lines hookup so when you pump the brake pedal it will shoot the fluid back into thereservoirs. You can also have someone pump up the brakes and hold them asyou would for normal bleeding, and then you loosen the line fitting at themaster cylinder, allowing any air to escape. There will be fluid too, socover any painted surface you do not want ruined by brake fluid. Thenretighten it before releasing the brake pedal. I believe you should do thefront line first, and it will probably take a few bleedings to get all theair out.

    Archivist: Tom Wilson

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