1983 Ford Escort Locking brakes

Hi I recently asked for help with my car and got a good and quick rsponse, my brakes are locking, I had replaced the master cylinder, and I was then told by one of you guys that there could be a leak on my wheel cylinder seals, I therefore replaced bith left and right wheel cylinders and brake pads, however the front brakes are still locking, the thing is though that they only start to lock up once the car has driven a bit, once the car is warm, My question therefore is, What could cause it to be locking up and building back pressure on the brake pedal?