Your master cylinder made for disc/drum brakes will have a residual pressure check valve in the drum brake side.
Maybe, maybe not. Actually not likely. An original master cylinder for drum/drum had them in both ports, but I've found that most rebuilds these days don't have them. Also a new aftermarket repro master cylinder won't have them either.
You can check to see if they are there. Put the smooth end of a small drill bit through the hole in the brass cone seat in the outlet port. Probe around and see if you feel rubber. If you do, that's the residual valve, it's a "duck bill" valve under the cone seat. To remove, run a 6-32 tap in the hole in the cone seat, then put a 6-32 x 1" long (or so) screw in the tapped hole. You can now put the claw end of a claw hammer under the screw head and pull the brass seat out. Remove and discard the rubber residual valve and the spring under it. Clean up any burrs on the cone seat and reinstall it.
A system that has a disc/drum master cylinder (big/little reservoirs) and a disc/drum combination valve has a residual valve in the outlet of the prop valve, none in the master cylinder. That's the primary difference between a disc/drum and a disc/disc combination valve. You can disable that residual valve too.