We have a 1966 Plymouth belvedere that we race in the 24 Hours of LeMons series that has a strange phantom brake issue. If you are driving about 5 mph and turn left,the brake pedal will go to the floor if you press on it and the car keeps going. We tried a new master cylinder and that did not help. It has been converted to front disc with a correct disc master cylinder and prop. valve. The brakes work fine otherwise. Any ideas????