Did you by any chance upgrade your brakes at all? my 70 Charger had 11x3 drum brakes up front and 10" drums on the rear. I swapped everything from a 75 Disc braked Dart on to the system except for the bigger booster and the proportioning block...I feather could touch the brake pedal and it would lock all 4 corners. Problem went away after a larger booster was installed and the correct prop valve