I have a 65 Plymouth Sport Fury that I built 15 years ago that ran great up until a couple of years ago. But after some major engine repairs last year has ran great ever since. But the last couple of times I have driven it I have noticed that when I applied the brakes the car does show down and then it gets to the stopping point the brake pressure goes away and the peddle goes to the floor but the car does stop. But a time or two I have had to use the parking brake to stop the car. Now the engine does have a vacuum recovery system for the brakes. Due to the cam that is in the engine the vacuum pressure is a little lower then it should be and the vacuum recovery system stored the vacuum for the brake system. What could the problem be.