Brake problem
 

Ok, I know it sounds pretty elementary , Brakes on an old don hardy chromoly chassis, frame mounted master , Lamb calipers, front, Willwood rears.
Car had a problem with locking up, when down at the end of 1/4 m.
We found the master had some crud in it, as it was under the car and close to the ground it picked up a bit of dirt, kida rusted up a portion of the inner bore of the master.
I replaced with the matching part, same situation, except the brake rod is returning all the way back but there is still brake pressure in the system.
I figured, possibly line lock, eliminated that issues, problem still persists.
Bled the darn thing 20 times, still no luck.
Anybody?

Brake
 

Which brakes are locking? I would replace the flex line to the rear brakes and the flex lines to the front wheels. A friend had a similar problem with a hydraulic clutch. The line had swollen internally and acted as a oneway valve. Replacing the line fixed the problem. Would also look for a crimped line and sticking calipers or wheel cylinders.

the front are locking, I went under and released some pressure from the master and it freed up. Starting to think there is ahydraulic lock in the master.

Brakes
 

Are you sure that the actuation arm from the pedal to the master cylinder is the right length? Seems that with the pedal all the way back, all the pressure should be off.

proportional valves..this is why they make in line wildwood inline residual pressure valves..check out these items..

That still wouldn't prevent the line pressure from releasing,since the residual valves actually hold pressure on the line.Not enough to cause a drag or lock up,but not having them wouldn't create his problem

I think we solved the problem.
The car has a remote pressure guage that runs down the length of the front frame, pretty close to the header. When we rebuilt the system, I did not realize I had to bleed the guage. Thus there was a big air pocket. As the car heated the air expanded and it somehow locked the master. We are going to give it a try tonight and see it it solves the problem.
Thanks for all the help.
Al