71 demon brake questions.

Somewhere I read about the early wheel cylinders on the rear didn't have a spring between the piston cups. The drum only master cylinders had compensator valves in them to maintain some pressure on the wheel cylinders to prevent fluid flow back to the master cylinder in order to maintain pedal height. The newer disc brake master cylinders did not have the compensator valve due to them putting a spring in the rear wheel cylinder to hold the seal/pistons apart. This in essence acts like a built in compensator. You may have to add that spring to or replace your wheel cylinders with newer year models.