Thanks for come backs:
'degins' hit the nail on the head: It seems that there is a pre-load on the master cylinder, or even just one of the caliper pistons, that is causing the brakes to be applied a small amount, all the time. The caliper(s) heats up from the constant braking and eventually transfers the heat to the brake fluid. The fluid volume increases with heat and the resulting increase in pressure causes the brakes to apply even more. This progressive failure will eventually lead to brake lock up.
I rebuilt the clapers/pistons/o'rings/dust covers a few months ago. Yesterday I bled all the old fluid from Mst Cyl (stock Mustang manual master cylinder for disk brakes) it was rust colored and cloudy. Pedal is good with about 3" travel when cold and wheels have slight brake drag when jacked up. After 20 minutes of normal driving at 75'F ambient temp, the pedel gets hard with almost no travel. Both front brakes when measured with an IR temp probe are up to 250'F, back brakes are about 150'F. Wheels are locked up! When the bleed valve is cracked fluid comes out. After about 30 minutes of cooling the pedal gets travel agin and the wheels can be moved with a some force. Brake temp by then is down under 125'F.
Since rust was found in the master cylinder could the problem be with the check valve seal, residual check valve, the piston return spring, primary cup piston or secondary cup? Brake pedal/rod to master cylinder have not been altered.
What about the proportioning valve? It seems to be a little rusty where some brake fluid may have escaped.