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Hot Rods master cylinder won't relieve pressure

Discussion in 'The Hokey Ass Message Board' started by wreckfixer, May 31, 2021.

  1. wreckfixer
    Joined: Jun 15, 2009
    Posts: 320

    wreckfixer
    Member

    I have a 57 Mercury. replaced the master cyl, all the wheel cyl. and rubber hoses. bled the system everything seemed fine. I drove the car around town and before I could get home the brakes where all but locked up. I broke the line loose at the master relieved the pressure and it happened again. any suggestions? All stock single port mc, manual brakes.
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
    Member

    you need free play at the pushrod, so the piston can move all the way back to uncover the relief hole.

    Do you have free play? or is the pushrod tight when the pedal is released?
     
  3. speedshifter
    Joined: Mar 3, 2008
    Posts: 312

    speedshifter
    Member

    Make sure the push rod that pushes on the mast cyl piston has a little free play when the brakes are not applied. Greg
     
    mrspeedyt and olscrounger like this.
  4. F-ONE
    Joined: Mar 27, 2008
    Posts: 3,271

    F-ONE
    Member
    from Alabama


  5. Wanderlust
    Joined: Oct 27, 2019
    Posts: 787

    Wanderlust

    Proper return spring on pedal?
     
  6. jaracer
    Joined: Oct 4, 2008
    Posts: 2,417

    jaracer
    Member

    I agree with everyone about having free play in the pedal. That's the first thing to check.

    Does it have power brakes? I had a Lincoln that was really hard on brakes. I could only get 10k miles before it needed a reline. One day at a friend's house while sharing a few brews, I noticed the brake lights come on on the Lincoln. Turns out that the booster would slightly apply the brakes from time to time. Replaced it and never relined the brakes again for the last 7 or 8 years I had it.
     
  7. squirrel
    Joined: Sep 23, 2004
    Posts: 55,931

    squirrel
    Member

    that's not the problem here
     
  8. Mr48chev
    Joined: Dec 28, 2007
    Posts: 33,861

    Mr48chev
    ALLIANCE MEMBER

    As Jim and a couple of others said, if the push rod is pushing the piston in even slightly it covers the compensating port and the fluid cannot return to the reservoir.

    I've seen it a couple of times when replacing the master cylinder the hole for the push rod in the replacement master cylinder's piston is slightly shallower than the one in the old one was. Brake master cylinder diagram yellow fluid.jpg
     
    mrspeedyt likes this.
  9. wreckfixer
    Joined: Jun 15, 2009
    Posts: 320

    wreckfixer
    Member

    Thanks for the help! I swapped the the plunger from old to new and that was the problem. Bam within 10 minutes you guys answered my question.
     
    mrspeedyt, Truck64, F-ONE and 3 others like this.

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