drum brake proportioning valve

a residual pressure valve is used with drum to keep the wheel cylinders from leaking fluid
the way it works is by maintaining a small amount of pressure on the seals so they stay expanded, without this pressure the seals in the wheel cylinders can sometimes contract enough to leak
modern wheel cylinders have a spring in them that presses on the seals and serves the same function as a residual pressure valve

proportioning valves are used with front disk, rear drum setup's in order to maintain proper front rear brake balance, they are not used with 4 wheel drum brake applications
on a 4 wheel drum brake, dual reservoir setup you only need the proper distribution block

I would check that your brake shoes and drums are clean and properly adjusted
1: with the wheel on turn the adjuster through the slot in the backing plate so that when you spin the tire by hand it will not spin more than 1/2 rotation when you let go

it is also possible that the master cylinder was mislabeled or defective, this happens quite frequently with re manufactured parts