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  1. #1
    Join Date
    Jul 2017
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    Nashville, TN
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    97

    Brake lockup issue

    So I am finally test driving my swap etc, 1978 Firebird with a GMPP LS3. I am having an issue where the brakes build pressure as I drive. All seems fine at cold start but as I drive the brakes seem to slowly get tighter and tighter until they eventually just lock up. I have wildwood dynalites with a dual bowl master cylinder running off intake vacuum. It seems I get vacuum but need to measure how much. Any advice on what needs to be done here or what is causing brakes to gradually lock?



  2. #2
    Join Date
    Sep 2010
    Location
    Beach Park IL
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    2,405
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    Incorrect booster to master cylinder pushrod clearance. .020" - .025" is what you want.

    Incorrect pedal to booster pushrod adjustment.

    Those two things are most likely the issue. The following is rare but something else to check after confirming the above.

    Brake lines to close to exhaust. Rarely an issue

    Brake hose failed and acting as a check valve.

    Residual valve failed and acting as a check valve.
    Donny

    Support your local hot rod shop!

  3. #3
    Join Date
    Jul 2017
    Location
    Nashville, TN
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    97
    So I adjusted the pedal to booster rod and all seemed good until they locked up again. The pressure subsides slowly as I sit and the car cools down but then when I drive they tighten back up again after a short distance even when I am not using the brake.

  4. #4
    Join Date
    Jul 2017
    Location
    Nashville, TN
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    97
    https://www.firebirdcentral.com/1967...p/brc-1021.htm
    Here is the link to the booster master setup I have.

  5. #5
    Join Date
    Jul 2017
    Location
    Nashville, TN
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    97
    So it was the booster to master cylinder rod. I had it set all the way back but the master was still being depressed. I used a few washers to space it out, drove for 20 mins or so and all seems good. Not real happy the master doesn't fit the booster right and they are sold together. Anyway thanks so much for the help.

  6. #6
    Join Date
    Sep 2010
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    Beach Park IL
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    Look inside the master cylinder, sometimes there is spacer or filler in the pushrod hole that can be removed and shortened.
    Donny

    Support your local hot rod shop!

  7. #7
    Join Date
    Jul 2017
    Location
    Nashville, TN
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    97
    I modified the spacer and got the plunger rods all dialed in where they need to be on both sides. Now my issue is pedal engagement. My pedal goes almost all the way to the floor before engagement. I have bleed the master and the brakes, no air anywhere. If I pump the brakes a few times it builds pressure but I don’t think pumping the brakes 4 times at every stop is a good option. I have Wilwood Dynalite 4 piston calipers front and back, stock disc/disc prop valve, 8” booster and 1”master. I am thinking the master may be bad somehow, even though new. When bleeding the master it seems the front has way too much pressure somehow as the front pot shoots fluid out of the bowl to where I have to place the cover over the reservoir to prevent a mess.


    Any thoughts on this setup or if this sounds like the master to anyone else? I would hate to drop another $100 plus on a new master and bleed everything for no reason.

  8. #8
    Join Date
    Sep 2010
    Location
    Beach Park IL
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    What is your booster pushrod clearance number?

    If the pedal pumps up, there is air in the system.
    Donny

    Support your local hot rod shop!

  9. #9
    Join Date
    Jul 2017
    Location
    Nashville, TN
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    97
    Rechecked and adjusted everything, booster pushrod clearance is .020, re bled master cylinder, and adjusted the pedal rod. Brakes are working well now, think I had air in the master. I did bench bleed, but then when I bled in car by releasing the line at the master some extra air came out. Didn't realize I could in car bleed as well as bench on the master without bleed screws.