Not sure how you would handle the pressure differential between disc vs drum.
Generally, drum brakes require about 400 to 500 psi, discs 800 to 900 psi. So the disc side of the master cylinder runs at about twice the drum side. Some modern disc run 1500psi to the discs.
Proportioning valve manages the pressure front to back in disc/drum.
If disc/drum master was used in drum/drum .........front drums would lock up prematurely and be super sensitive.
