How come all my manual drum cars have disc master cylinders?

The master cylinder installed is correct. The correct disc brake master cylinder would look almost identical except for the casing is deeper to hold more fluid. That one is quite rare too so part stores would hand over a later model master cylinder per their interchange books. The one difference in this interchange that some run into is the pedal rod length difference. It's more correctly a difference in the length of master cylinder castings. They would need to readjust their brake lamp switch position. I'm going to guess the current position of your brake lamp switch is preventing full return of master cylinders piston. Could be why no fluid out to rear. Just a guess.