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 https://www.v8buick.com/index.php?threads/metering-valve-hold-off-valve-button-function.379880/) 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 musclecarresearch.com 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?
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?
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?
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.
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
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.
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.
Yes, also new. When I release the brake pedal the rear wheels immediately run freely again. Don't expect an issue there.
Is the break light on? The proportiong valve can be stuck in the back breaks work only position. Just a guess and good luck
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.
Door etc bolted on, bleeded once more and no more locking, so I think it is ok now. The pedal feel is different than I remember though, but I think it makes sense. The first bit, 1/2 inch maybe, it feels a little soft and then it feels normal. Is that the pedal travel of the first 30 psi before the hold off valve opens? Do people with this setup recognize this? I think the hold off never worked for me, so this is new for me.
If you have new brake pads on it, it may take a few miles of stops before the pads bed-in. Usually the stopping gets progressively better. My disc brake kit came with instructions for the sequence of stopping to bed the brake pads in properly.