I have a '64 3.8 OTS. I had the master cylinders sleeved by Apple Hydraulics and calipers sleeved by White Post. When I get in the car and press the brake pedal it is nice and firm. I drive it 30 feet and press the pedal and it goes halfway to the floor. The problem is with the front brakes. I can’t find any leaks. As I continue to drive, each time I want to stop, the pedal goes halfway down the first time I press and is firm on the second press. I cannot repeat this situation parked in the garage. It only happens when I drive it. Any thoughts?
What happens if you disconnect the vacuum hose from the booster and plug it? If the problem goes away, then the booster is at fault. If not, it is a mechanical or hydraulic problem with the master cylinderw.