[68gmsee]

Many years ago, I had the same problem with rear brakes. In my case it turned out to be a severely swollen (internally) rear rubber brake line. Pressure would allow brake fluid to the brakes but there was not enough spring pressure to allow fluid to return so the brakes stayed engaged until they cooled and gradually returned to normal.

Rubber lines will swell up after years of use and you can't tell by looking at them from the outside. I replaced them all to be on the safe side.

To determined if that's the cause loosen the brake bleed valve when they're locked to see if they loosen up. Otherwise, check the brake booster and push rod as 67 chevelle mentioned.