I posted about a month ago about the brakes on my 69. All brakes were locking up. I replaced the master cylinder & now the rears are the only ones still giving me trouble. It does have power brakes. I don't believe they are stock. I can drive it for about 10 or 15 minutes or let it sit for 20 to 30, either way the brakes lock up tight. I can turn the engine off & let it sit for half an hour & they release. The immediate fix is to crack the rear line from the mc & bleed out some fluid. They release instantly.
The guy I bought it from was the only owner to have the brakes lock up( I have spoken with the 2 owners prior to him). He had the rear brakes worked on after that. He also had new exhaust installed from the headers back. I'm not sure of the timing in relation to the brake issue.
One thought is that the wrong fluid is in the system. I put dot3 in when I changed the mc. Is there a chance that the fluid is boiling? I have noticed an extreme amount of heat when they lock up. The rear line coming off the mc is too hot to touch. The front line is warm but not hot.
Another suggestion has been that the rubber lines are bad & not releasing pressure. Would that happen w/o ever applying the brake?
The last thing I can think of is the proportioning valve is stuck in one position. Again, would this happen w/o pressing the brake pedal?

 

Your rubber lines are shot. Common problem, they degrage from the inside and hold return pressure. Install new flex hoses.

 

You may have a bad Master Cylinder, incorrect length MC rod or defective flex lines.

The extreme heat issue is caused by the brakes not releasing. The combination/proportioning valve will not cause your brakes to lock-up, it only selects the pressure routing to both front and rears or isolates a leaking system. If actuated then you should have a warning light illuminated on the dash.

I would jack up the rear of the vehicle and put the car on jack stands. Two person job; have assistant apply brakes while you are turning the wheel by hand. Wheel should stop when pressure applied, and while keeping force on the wheel to rotate have assistant release the brake pedal. The wheel should freely rotate immediately when the pedal is released. If it hangs up at all you may have internally ruptured brake flex lines or a malfunctioning Master Cylinder which is supposed to relieve the brake pressure or the brake actuation rod between the master cylinder and brake booster is not the correct length.

If the system worked before you replaced the Master Cylinder, I would suspect that you installed a faulty one.

 

Would the lines hold pressure when I don't even apply the brakes? I can crank it up & leave it in neutral, never touching the brake pedal & the brakes still lock.

 

If your rubber lines collapse (on the inside) then they will hold the pressure on the pads causing your problem.

 

the rod that goes into the MC is adjusted too long and is preloading the MC

 

I know the rod isn't too long. I purposely adjusted it too short to eliminate that possibility.

 

Thanks for all the help guys. I finally got it working. It was the pushrod. It was loose & adjusting itself. We got it adjusted correctly & whacked it with a mallett just hard enough to hold it.