Brake pedal goes to the floor when car is started

With the car off I have a firm pedal that does not drop over time with constant pressure. As soon as the engine is started the pedal goes to the floor. When I turn the engine off the pedal pushes back up and stays firm again. Have installed and adjusted rear drum brakes with new cylinders and shoes. Have installed new front rotors and checked calipers (no leaks) and pads. Bench bled and installed new mater cylinder Bled all four corners with a pneumatic bleeder and pulled 3 quarts of fluid through the system. No fluid leaks. Car will stop but no matter how hard I push the car will not brake hard. This has been the same issue since I bought the car. Front brakes are disc from a 1976 Chrysler, rear are stock drum. Booster, master cylinder and combo/proportioning valve are not stock, believe there are from a 1970's Chrysler a.