Brake problem

[Jakke]

I have 73 B-body

I rebuilt brakes; new lines and hoses. I cleaned pistons and changed new seals to calipers.
And I changed new parts into master cylinder.
Problem is that after few minutes driving brakes will not give up. I have to release pressure from calipers..

[DartGT66]

Check the brakehoses, they may be blocked because of internal structure failure, and leave the pressure on.

[Ray Bell]

New lines and hoses... that shouldn't be the reason...

Does the new master cylinder buildup have the drum brake pressure retention valve fitted? This isn't required with discs, of course, and does exactly what you're describing.

[John Kunkel]

This problem is often caused by the rear piston in the master cylinder not fully returning, this covers the relief port in the rear reservoir and prevents the fluid from returning to the reservoir.

With the cover removed, slowly depress the brake pedal and look for a burble in the surface of the fluid. If there is no burble, the relief port is blocked. If the car has power brakes this can be fixed by adjusting the push rod at the front of the booster.

[Shaun]

Do you have a proportioning valve? If you do I would agree with John-your rod is out of adjustment.
Shaun

[BBABODY]

I have a barracuda that did that to me. Took it to a shop and they told my it was the calipers. So I changed them. Didnt do a thing. Brakes still would pump up and not release. So I went to another shop was told it might be the push rod out of adjustment. Pulled the push rod out to find out its non-adjustable. I got pissed then and took a ball peen hammer to the master cylinder and the brakes freed up. Maybe try a new master cylinder.