Master cylinder cap leak from hell!

Discussion in 'Suspension, Steering, Brake & Wheel Topics' started by 1974blackz28, Aug 1, 2019.

  1. Gary S

    Gary S Administrator Lifetime Gold Member

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    I've always bled brakes without the cover on the master cylinder. I just lay a rag over the master to stop the minimal spray that might come back. Usually the rag barely gets wet. It is much easier to monitor the level during bleeding with a loose rag in place than with the cover snapped down.
    I wonder why you have that much back pressure when the pedal is pushed and released. It seems excessive to me. Mine don't seem to do that.
     
    Knuckle Dragger likes this.
  2. cadillac_al

    cadillac_al Veteran Member

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    This is a weird one. It makes no sense to me. The MC cap is pretty low tech. Are you getting fluid to the back brakes? If it's an adjustable prop valve I would open it all the way for now while bleeding. It sounds like the MC reservoir is getting pressurized but I have no idea why.
     
  3. 1974blackz28

    1974blackz28 Veteran Member

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    Ok all.

    Update...

    I changed the master cylinder AND brake booster with what was in the autozone catalog for the car. AC delco 54-71110 was the booster part number.

    Same problem!!!!! Also I cannot get the brakes to feel anything less than spongey! I cant get the wheels to lock at all
     
  4. Gary S

    Gary S Administrator Lifetime Gold Member

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    I feel that you shouldn't have that much fluid spraying back into the master cylinder. As I stated in post #21, I usually just place a rag over the master cylinder when bleeding and the fluid coming back barely gets the rag wet. But, you do have a different system after the master cylinder. The change in valving might make that difference as well as the rear brakes being disc brakes.

    Are you getting fluid out of all of the wheels when bleeding. If not, you could simply have a blockage down the line. If some of those lines are filled with air and blocked, then the compressed air will blow back when released. That would also explain the spongy brakes.
     
  5. mallard

    mallard Veteran Member

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    What an odd problem. Cadillac Al’s post probably makes the most sense. There must be some kind of issue with that proportioning valve. I remember in my mechanic days we had grief with proportioning valves on Fords getting them “on centre” so that fluid traveled to the front/rear properly.

    I’m super curious to learn how this turns out.
     
  6. 1974blackz28

    1974blackz28 Veteran Member

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    Update Number 2:

    I put the 1979 Corvette Master cylinder which was designed for disc/disc vehicles on the car. The leak is Gone!!!!

    BUT

    I cannot get a firm pedal at ALL. When I bleed the brakes, fluid comes from all four corners. The pedal will get stiff on lets say for example wheel number 2 being bleed. Then when I move to wheel number 3, it all goes south and the pedal feels soft.
     
    John Wright likes this.
  7. Gary S

    Gary S Administrator Lifetime Gold Member

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    Failure of the pedal to get firm is usually air in the lines. Just because you get fluid out of each wheel doesn't 100% say that there still isn't some air trapped in a high spot in the lines. If the air is sitting in a high spot, the fluid doesn't always push it through the system, but instead it can bypass the air leaving your problem. Continued bleeding should eventually get that air out. Make sure all your bleeders are tight when bleeding and releasing the pedal because if they are even slightly open, releasing the pedal pulls some air back into the system.
     

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