Had one that would lock up the brakes as it warmed up, on a 64 Chevy 1/2ton, along time ago. I was on the freeway on the way to work early in the morning, and the truck started to slow down and gradually came to a stop. Sat for a while, and then was able to get off the freeway, then it came to a stop again. Took a while to figure out what was up. Put a new master cylinder in on the side of the road and got to work late. The piston had crumbled slightly and the debris would inhibit the movement of the piston. You might have some dirt/rust in the bore?
