Drum Brakes Locking Up (dont have a brake booster)

Discussion in '40- & 55-Series Tech' started by kylenine, Nov 7, 2013.

  1. kylenine

    kylenine

    Messages:
    78
    Location:
    Los Angeles
    Making a long story short....
    I have a 1967 FJ40, got it running, went to bleed the brakes and that is where all trouble started...

    I replaced the front with a 1973 FJ40 axle. I replaced all brake cylinders and shoes with brand new ones and also put a dual reservoir brake master cylinder for safety. I drove down the road and the brakes worked great but after a while the pedal became too hard. I had to get out and back off all of the wheel cylinders and managed to crawl home. I have adjusted both the push rod and the wheel cylinders multiple times and it always ends up locking the pedal up.

    I have bled the whole system, I know how to adjust the wheel cylinders, and my brake master cylinder does not have a brake booster attached so it definitely isnt the "brake booster push rod."

    When it is cold, the pedal goes to the floor but after driving for around 5 miles the pedal is rock solid and my brakes get stuck on. I am going to bleed the whole system through again tomorrow and check for air bubbles. I definitely dont have any leaks. I know that there have been threads about this and I have read them but they all talk about adjusting the brake booster push rod and I dont have one. Is it the adjustment of my normal push rod that needs to change? How do I measure it?

    Does anybody have a solution to this?
  2. JohnnyC

    JohnnyC Long ago TLCA# 2231 SILVER Star

    Messages:
    11,021
    Location:
    Albany, NY area
    yes normal push rod needs adjusting... or your missing spring
  3. Even though you don't have a booster, there's still a rod that pushes into the master from the linkage. If that rod doesn't pull back enough, the piston won't withdraw all of the way and you'll end up with residual pressure. Make sure that rod has play in it.
  4. Fast Eddy

    Fast Eddy SILVER Star

    Messages:
    11,614
    Location:
    Morgan Hill, CA
    This is not normal, no matter how the push rod is adjusted. Sounds like your drums are not adjusted correctly or you don't have the proper residual valves in the master. Fairly dangerous if your first brake push in the morning happens to be at a busy intersection.
  5. I suspect that when the rod is adjusted properly, the lockup will end. Then the cylinders will likely need adjustment so the pedal doesnt go to the floor.
  6. kylenine

    kylenine

    Messages:
    78
    Location:
    Los Angeles
    The spring is on the pedal.

    I have adjusted the push rod multiple times, is there a certain measurement that it should be?

    I may have been wrong about the "cold part" because this morning it was about 50 degrees and it actually felt nice. I didn't drive it today though.

    I adjusted the drums by jacking the wheel off the ground, adjusting the shoes til they hit then backed them off 3 clicks. They all feel nice until I start driving for a long time then they seem to get heated up. Could my master cylinder be bad?
  7. Fast Eddy

    Fast Eddy SILVER Star

    Messages:
    11,614
    Location:
    Morgan Hill, CA
    The order of adjustments/bleeding has to be right. Adjust the pads first, bleed, adjust pads again, then you do the final adjustment of the rod. There is a spec. I don't remember what it is, but you want a very small gap between the rod and the piston in the master. It's something like 0.1mm. Not sure, but really small like that. You need a caliper to measure the depth of the master and also a small straightedge to measure how far the rod comes out past the firewall and gasket. Once it's set up, you can't mess with the pedal spring or adjustment, so they has to be set up first.
  8. sggoat

    sggoat

    Messages:
    1,546
    Location:
    Alva, Fl.
    If you go out in the morning and start the truck(actually, you don't even need to start it since you don't have a booster), and step on the brake and the pedal goes to the floor---you have a leak.
    If as you say, there is no evidence of fluid on the ground(and the reservoir is still at normal level(or thereabouts) the leak is internal to the master cyl.
    After the rig runs for a while, the heat builds up and the "leak" seals off--need to check the master cylinder bore - cups and pistons.
  9. If your brakes are locking up over time, it's because you have too much residual pressure in the line. Until you deal with that, you can't accurately adjust the shoes because every time you step on the brakes to test them, they move in a little bit and stay there.

    There are two things that can cause residual pressure. The most common one is that the pushrod is adjusted too long. If you can wiggle it and feel a little free play then it's not too long. If you can't feel any free play, adjust it until you can. The other cause of residual pressure is if the residual valves are wrong for your brakes. There is supposed to be a small amount of residual pressure in the lines with drum brakes, but it should be less than the strength of the shoe return springs. If it's too high, the shoes won't back off. What is this master cylinder from?

    Finally, the correct adjustment on the shoes is to tighten them until you can't turn the wheel anymore and then back off 2-3 clicks. When you are done, they should still drag but you should be just barely able to spin the wheel one revolution with a lot of effort. Is that what you did?

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