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Technical Brake drag when hot.

Discussion in 'The Hokey Ass Message Board' started by Carl Wurfel, Apr 29, 2020.

  1. Carl Wurfel
    Joined: Nov 9, 2017
    Posts: 68

    Carl Wurfel

    I am having a problem with brakes on my 55 Chev 3100. Brakes are a kit from POL performance online.
    kit mounts under the cab 55 2nd series. Booster and Master cylinder with the proportioning valve disc brakes on front drum on back all basically 1968 Camaro brake setup as they told me.
    I drive 1/2 hour and the brakes come on, I thought it was a vacuum problem so I tried putting a vacuum canister into the setup but I have enough vacuum pressure.
    I took apart and adjusted the booster nut all the way in to re leave any preload.
    I tested and brakes came on again, after about 20 min they release once cool.
    I then put shims in between the MC and booster about 5mm, I am not able to see any difference problem happened again so I have the adjustable vac booster nut to max plus I have a 5mm shim installed and still have the same problem.
    Im not sure I am on the right track but seems to lock up after a drive.
    my exhaust pipe passes near the MC setup too to add some heat.
    anyone have any other ideas? thank you
  2. olscrounger
    Joined: Feb 23, 2008
    Posts: 3,849


    is the rod needing adjustment?--shorten it a bit maybe. Have had this issue a few times and it was the rod length
  3. Everything clean inside the brake lines ? I had a similar problem with my '63 Falcon Sprint and it turned out to be a small dirt particle in the brake line at the master cylinder. After a 1/2 hour drive ( same as you ) the brakes stayed on, brake-lights too. Could,nt see it at the pedal though. The lines stayed under pressure. Let the car stand for 20 minutes and the pressure was gone. Dismantled and flushed everything and it was OK.
  4. Same here, but he said he shimmed the distance between the master and booster to effectively shorten the distance. May still be too long? Maybe check with the kit manufacturer to see if you have the right parts.
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  5. Doublepumper
    Joined: Jun 26, 2016
    Posts: 609


    One other thing to check, verify there is between an eighth and a quarter inch of free travel at the pedal.
  6. Keep track of any adjustments in case you have to undo them. But too little clearance in the rod to master can result in drag.
  7. Kevin Ardinger
    Joined: Aug 31, 2019
    Posts: 174

    Kevin Ardinger

  8. DDDenny
    Joined: Feb 6, 2015
    Posts: 13,450

    from oregon

    Had to do this when I put disc brakes on my 66 Suburban, I used components that weren't married together originally, on the first test run down the freeway, took the exit and coming to the stop it started slowing down on its own, wheels were really hot.
    The intermediate rod between the master cylinder and booster that I had originally made was an 1/8" too long, shortened it and the overheating/dragging issue dissapeared.
    Desoto291Hemi and olscrounger like this.
  9. rustydusty
    Joined: Apr 19, 2010
    Posts: 1,554


    Not hard to tell if your rod is too long, wouldn't be any play at the master cylinder...
  10. 67drake
    Joined: Aug 8, 2008
    Posts: 64

    from Avoca WI

    I’ve never had this issue myself, but 95% of the time it seems to be the incorrect rod, especially after aftermarket parts are used.
    olscrounger likes this.
  11. BJR
    Joined: Mar 11, 2005
    Posts: 6,355


    If the rod is too long the relief port is blocked and pressure builds up in the lines. When it happens crack a bleeder and see if fluid shoots out. If it does you are building up pressure in the lines. The only other thing I can think of other then the rod from the pedal to the booster or the rod from the booster to the master being too long, is a bad rubber line disintegrating and acting as a one way valve. I have seen both issues happen with the same symptom.
  12. chevy57dude
    Joined: Dec 10, 2007
    Posts: 5,548


    Same problem was making me insane on the '57.
    The rod from the pedal wouldn't stay in the cup of the M/C piston when releasing. Rod would touch the bottom of the piston and stay there, just enough to drag, but not lock the brakes. Let it sit and the pressure would bleed off slowly, until driven again. Made a little guide to keep it centered.
  13. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,628

    V8 Bob

    With the booster disconnected, make sure the pedal can move farther back than needed to allow full booster input (control) rod return.
    If you don't already have a pedal return spring, install one.
    The measurement of the booster output rod (or pin) to the master cylinder primary piston is critical for minimum pedal movement and full master cylinder stroke. If no instructions are available, I would use .040", and this is with full vacuum applied to the booster, or about 20"HG.
    olscrounger likes this.
  14. tractorguy
    Joined: Jan 5, 2008
    Posts: 642


    Very, very good advice. Especially with new components.......there can be debris or a snotty kinda gel material that constricts or floats in the lines. It may actually be from manufacturing, or machining. It can cause brake "pull" because debris is in a specific line going to one of the four wheels and it blocks the pressure, causing it to pull to the other side.
    As stated, it can also hold residual pressure in a line, causing the brakes to drag and then drag more as the heat in the drums/discs/shoes/pads built up.
    Flush everything......including taking the bleeder screws out for flush since debris may not pass thru bleeder screws.

    One last sure that if you have a brake light switch that has a plunger mounted against the pedal, that it is not mis-adjusted, keeping the pedal from totally returning to rest position.
  15. Carl Wurfel
    Joined: Nov 9, 2017
    Posts: 68

    Carl Wurfel

    Thank you very much, another person mentioned today that because my system is mounted lower than the brakes I need check valves between the brakes, 2psi for front and 10psi for rear.
    so I need to try that.

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