Problem: rear wheels lock up easily during braking '69 GS

Discussion in 'The whoa and the sway.' started by srb, Aug 14, 2022.

  1. srb

    srb Well-Known Member

    I'm in the process of restoring my car and now working on the brakes.

    I have a disc/drum powerbrake setup with the old school hold off valve (see

    I have new lines, new hoses, new rear wheel cilinders, new master cilinder, new power brake unit.
    I had my hold off valve rebuilt with a rebuild set from and even today opened it up again to extra check. With modest air pressure, the valve opens, seems to work ok (still don't understand the button function though)

    I have followed the manual for the rear brake setup with almost binding and then 30 notches backing off. I have followed the manual for the parking brake, locking at 3rd click.

    I have bled my system now 4 times. Brake pedal feel is firm. It does brake in front, but with moderate braking, both rear wheels lock up.
    My car has no doors, no trunk lid, no bumpers, no window glasses. Not sure if that would make such a difference, but it feels like the rear is locking up way to0 easy.
    What am I missing?
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The car is under weight, so the brakes lock up easier
    Waterboy and Max Damage like this.
  3. philbquick

    philbquick Founders Club Member

    I would put an adjustable proportioning valve in the system, then you can adjust the lock-up as the weight of the car changes. I have never heard of backing off 30 notches, what is the purpose of that?
    Max Damage likes this.
  4. buick64203

    buick64203 Supercar owner Staff Member

    You adjust the rear brakes till the shoes barely touch the drum.
  5. Guy Parquette

    Guy Parquette Platinum Level Contributor

    It is possible that the rear shoes/ drums are contaminated with brake fluid. But usually happens just on one side.
    The contamination will lock the wheel with just slight brake effort. If that’s the case, the only fix is replacing the shoes and cleaning the drum. Maybe even turning the drums as the fluid will migrate into the pores of the porous cast iron drum.
    Of course also finding and fixing the leak. In my case it was both new brake cylinders.
    Maybe they sat too long thus drying up the seals?
    Last edited: Aug 14, 2022
  6. Ziggy

    Ziggy Well-Known Member

    Check the size of the rear wheel cylinders. I have seen case where cylinder that are too big in diameter exert excessive force for a given hydraulic pressure. Cylinders are available in a variety of diameters. Also, make sure the master cylinder piston is the correct diameter to guarantee correct feed pressure. I have also seen mismatched friction coefficients between front and rear pads and shoes cause imbalanced braking.
    Guy Parquette likes this.
  7. Guy Parquette

    Guy Parquette Platinum Level Contributor

    Not to have you overthink this, but smaller diameter bore and piston cylinder will react sooner or quicker than a larger bore with less pedal travel with the smaller bore. Applying the braking force sooner than with a larger diameter bore cylinder.
    I went through all this while converting to four wheel disc brakes on my race car. Using all factory GM parts.
    If you’re using all stock replacing components. Disregard all of this! Lol
    Last edited: Aug 14, 2022
  8. GS44667

    GS44667 Worlds First Stage1 Conv

    All good points
  9. srb

    srb Well-Known Member

    Wasn't the case when I disassembled it. Brake shoes look fine. Also correctly orientated.

    Bone stock sizes. 7/8" for the rear and 1 1/8" for the MC.

    At first I figured that most of the weight that will be added wil be contributing more to the front than the rear so that it wouldn't matter much. But it still is a significant amount of weight, so I start to think you are right. I'll leave it as is first and build the rest in.
  10. 69 GS 400

    69 GS 400 Well-Known Member

    Looks like you have replaced the majority of the parts. Have you replaced the hose that splits off to each side on the rear brake lines on the rear axle . Those can collapse over time and act as a check valve. You can't see anything by looking at it. I spent an entire long weekend pulling my hair out over that one on my car.
  11. srb

    srb Well-Known Member

    Yes, also new.
    When I release the brake pedal the rear wheels immediately run freely again. Don't expect an issue there.
  12. 67melrob

    67melrob Member

    Is the break light on? The proportiong valve can be stuck in the back breaks work only position. Just a guess and good luck
  13. srb

    srb Well-Known Member

    No it's not. I have new tires with more grip, that's helping. And I have more stuff bolted on, which is also helping. Still waiting for the doors and decklid and hood, so I think Larry is right.
  14. pbr400

    pbr400 68GS400

    Try filling the trunk with heavy stuff and see if it gets better.

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