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

As long as you keep both chambers full and have removed the residual valve to the discs, ANY M/C will work. The dual chamber size is a nod to lazy people who never check their fluid level.

If you cannot get fluid coming out from the chamber that supplies the rear brakes, which should be the one furthest from the firewall, then either;
1) the mc has not been properly bench bled, or
2)the compensating port is not open, or
3)the dang thing is defective.

#2 could be due to a wrong length pushrod, go look for the fluid fountain after releasing the brakes, which indicates returning fluid. Watch out!!, the fountain can be a geyser if you have air in the system: and with brake-fluid everywhere on the paint, you have a bigger immediate problem.