master cylinder upgrade

No, master cylinders for drum brakes have residual valves to keep a small amount of pressure in the system so air does not get drawn in past the seals. A disk mc doesn't have the residual valve on the disk side because there is no spring at the caliper to force the pads away from the disk when your foot is removed. Also, a disk/drum mc will not have equal size reservoirs. The disk side is larger because as the pad wears the piston moves out of the caliper requiring more fluid. They are sized so the fluid in the reservoir does not run out before the pads are worn out.

IMHO, I wouldn't replace a functioning mc until you decide to do a upgrade to disk brakes then get the proper mc for the disk system.