Brakes locking on

Have a 68 Dart with the complete Dr Diff disc brake setup.13 inch front rotors and 11.7 rears.15/16 2 bolt aluminum mater.All new stainless brake lines all braided flex hoses and a drum brake distribution block(recommended by Dr Diff for this setup)Manual brakes.Problem I have is after driving the car for 1/2 hour to 45 minutes with some spirited driving and braking all 4 wheels have the brakes lock on solid and fluid comes back up the master cylinder caps.Let the car sit for 30-40 minutes and its fine again.I CHECKED AND THE BRAKE PUSHROD ALLOWS THE MASTER cylinder PISTON TO RETURN AGAINST THE C-CLIP RETAINER.It almost seems like the system builds pressure every time the brakes are applied and when it reaches a certain point the brakes lock on. Any ideas??????????