Most disc /drum master cylinders have built in line valves that maintain a small amount of fluid pressure in the rear drum circuit to keep the wheel cylinder rubber cups out against the sealing surface of the wheel cylinder.
To check, use a small probe like a tooth pick to gently push into the outlet port (rear port) on the master cylinder. If you feel a small rubber valve in there that compresses slightly when pushed on, the master has a line valve installed....
You can remove the valve by screwing in a sheet metal screw into the outlet port fitting and using a small slide hammer to remove the port...the valve and spring will fall out,,,,lightly tap the port fitting back on and remove the screw....
I would prefer to use a specific disc/disc master cylinder and an adjustable proportioning valve.
Another thing to consider is the correct piston rod depth in the master cylinder to match the piston rod in the booster.....
Good luck
