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brakes pressurizing when parked hot

Discussion in 'The Hokey Ass Message Board' started by Bmac40Ply, Sep 30, 2012.

  1. Bmac40Ply
    Joined: Nov 28, 2011
    Posts: 47

    Bmac40Ply
    Member

    Today I drove my 1940 Plymouth for the 1st time. All went well, except after a drive the brakes would pressurize and apply the brakes, and even turn the brake lights on. Then trying leave again they would really drag at first, but would eventually free up. They would also free up after time and everything cooled off. It has the original brake system and I just replaced all rubber lines, rebuilt wheel cylinders and the master cylinder. I have researched this problem and it seems it could be the push rod length is wrong or the return passage is blocked. My question is, when i rebuilt the master cylinder, the new piston i got was about a 1/4 inch longer than the original. Could this be blocking the return passage?
    Pumping the brakes before getting out also seemed to help them not pressurize and stick as bad.
    My other question is, I filled the master cylinder completely full, should have I left a little room for the fluid to return?
    Thanks for any help.
     
  2. delco666
    Joined: Aug 8, 2009
    Posts: 22

    delco666
    Member

    it seems to me if it were the wrong piston the return would be blocked hot or cold. i know from personal experience you must leave a little room in the mastercylinder or the brakes will stay applied.
     
  3. Bmac40Ply
    Joined: Nov 28, 2011
    Posts: 47

    Bmac40Ply
    Member

    works fine when cold and cruising.
    Only pressurizes after stopping.
     
    Last edited: Sep 30, 2012
  4. Mr48chev
    Joined: Dec 28, 2007
    Posts: 33,861

    Mr48chev
    ALLIANCE MEMBER

    if you push the pedal down with your hand how much slack do you have before you feel the pushrod push on the piston? If you have the pushrod adjusted out too far it won't let the piston and cup uncover the port that lets the fluid flow back into the reservoir of the master cylinder.

    And from a guy who has been working on old cars since before they were old trust me, the parts book isn't always right.
     

  5. Bmac40Ply
    Joined: Nov 28, 2011
    Posts: 47

    Bmac40Ply
    Member

    I had a hell of a time finding a rebuild kit for the master that included the piston. The car is in a different city now, so I cant go check. I plan to drive it here next weekend. I dont want any issues during the drive. It only seems to happen when i stop somewhere for a few minutes, but is fine while cruising.
     
  6. Push rod is too long now.
     
  7. Dane
    Joined: May 6, 2010
    Posts: 1,351

    Dane
    Member
    from Soquel, CA

    Sounds like the pushrod is applying brakes all the time. You have to have a gap between the pushrod and master cylinder piston or that happens. I set them up with a 1/8" gap.
     
  8. Bmac40Ply
    Joined: Nov 28, 2011
    Posts: 47

    Bmac40Ply
    Member

    Thanks for the help so far guys. I'll have to check that when I get back this weekend to work on it.
     
  9. Bmac40Ply
    Joined: Nov 28, 2011
    Posts: 47

    Bmac40Ply
    Member

    I can't go out and check the push rod length since the car isnt here, but I remember pushing the brake pedal have way or more before the brakes engauged.
     
  10. there is a VERY little hole that comes uncovered by the piston when you release the brakes. make sure it is clear of crap and that this 1/4" longer piston is uncovering it

    i have a friend who had this problem and it turned out to be crap in the brake fluid plugging that little hole
     
  11. daliant
    Joined: Nov 25, 2009
    Posts: 692

    daliant
    Member

    Filling the master right to the top isn't helping the situation either.
     
  12. 54MEB
    Joined: Nov 21, 2007
    Posts: 107

    54MEB
    Member

    I remember sitting on the side of I-45 in Houston grinding the pushrod shorter on the concrete so we could get a '34 ford truck back to the shop. The rod was just barely touching the end of the piston and covering that hole as mentioned. When the fluid in the lines got hot it built enough pressure to apply the brakes. You may be getting some under hood temps rising when it's parked hot and then you have the same effect. You want to be able to feel that little bit of slack before the rod touches the back of that piston. Good luck when you get back to the car!
     
  13. LOW LID DUDE
    Joined: Aug 16, 2007
    Posts: 1,223

    LOW LID DUDE
    Member
    from Colorado

    Don't drive the car any distance with the brakes dragging.Friend of mine burned his 50 Merc real bad ,later found out his brakes were dragging caused by master cyl. It heated up rear drum resulting in FIRE ! These guys have good advise on here. Check everything twice before heading out.
     
  14. sunbeam
    Joined: Oct 22, 2010
    Posts: 6,213

    sunbeam
    Member

    Could be a bad residual valve.
     
  15. Bmac40Ply
    Joined: Nov 28, 2011
    Posts: 47

    Bmac40Ply
    Member

    The master cylinder rebuild kit came with a new residual valve.
     
  16. birdman42
    Joined: Jan 18, 2012
    Posts: 400

    birdman42
    Member

    Might want to check the distance between the exhaust pipe and the brake lines.I have a friend with a 40 ford that has the same problem and he put header tape on the line and helped.He plans to move lines this winter.
     
  17. 40FordGuy
    Joined: Mar 24, 2008
    Posts: 2,907

    40FordGuy
    Member

    The guys are right, re; the pushrod, and piston mismatched....Be sure there is a return spring on the brake pedal to assure the pedal isn't keeping the rtn port covered......

    4TTRUK
     
  18. Bmac40Ply
    Joined: Nov 28, 2011
    Posts: 47

    Bmac40Ply
    Member

    Appreciate all the help, should be able to figure it out with the information given. Won't get to work on it until Sunday though.
     
  19. plym_46
    Joined: Sep 8, 2005
    Posts: 4,018

    plym_46
    Member
    from central NY

    2x on the relief port inthe bottom of the MC reservoir. 2x on the pedal free play should be an inch or so before it engages the the piston. @x on the do not over fill the reservoir, it needs some space for expansion of the fluid.
     
  20. Bmac40Ply
    Joined: Nov 28, 2011
    Posts: 47

    Bmac40Ply
    Member

    Got the brakes sorted out yesterday. Turned out that the new longer piston was blocking the little return hole in the master cylinder when at rest. My grandfather is a retired machinest, and decided that we just need to plug the old return hole with a set screw and drill a new return hole a half inch out further. We did this and put it back together and it all works fine now. Today the ol Plymouth made the 102 mile journey from where I had it stored and been working on it to my garage where I live. It ran flawlessly and was a great drive. Thanks for all the help!

    (If I knew how to post pics I would, Sorry)
     

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