Your brake pedal pushrod going into the MC is too long or doesn't have enough play.
Adjust it with about 1/16" or 1/8" play and the problem will mostlikely be gone.
Good call there, and I was just getting ready to mention that this can happen if the rod is overadjusted.
(The fluid can't return because the piston does not come back far enough to let the fluid return, and when they start getting hot you have a hydraulic lockup)