I replaced the master cylinder and all four wheel cylinders on my 61 Ranchero, bled the brakes and everything felt fine. After driving for less than 10 miles the brake pedal kept getting higher and I could feel drag on wheels. Eventually the pedal could not be depressed at all and the brakes were locked. I released some brake fluid from the right rear bleeder valve and the brakes unlocked. Another test drive and they gradually locked up again. Anybody have any ideas as to how to fix this problem?