The only difference between a drum/drum, a disc.drum, and a disc/disc master cylinder is the residual valves. Drum brakes have them built in, and disc brakes don't. If you want to run drums with a disc brake master, install external residual valves.
An easy way to check the master for residual valves, try to insert a toothpick in the fitting on the master cylinder.If it goes right in the cylinder bore, there is no residual valve. If it hits something about 1.4" in, it has a residual valve.
If you have a firewall mounted master cylinder, you might get by with a disc master cylinder, with no residual valves, and drum brakes