I think much of the confusion is coming from the lack of distinction between "distribution blocks", "proportion valves" and "combination valves".
THEY ARE NOT THE SAME THING.
A "combination valve" is usually a distribution block that includes a "proportion valve" and sometimes a "residual valve". The "combination valve" also includes a "brake light switch" and a "metering valve".
If you have drum/drum after about 1967 with dual MC it will have a "distribution block" with a "brake light switch" and NOTHING else inside of it. It separates the front and back system and turns the brake light ON if either fails. Nothing else.
A combination valve does a pressure dance as the pressure rises from 0 psi up to 1000-1500 psi. The "metering valve" restricts pressure to the front disc until the rear sees 50 psi. From 50 psi up to about 200 psi the front and rear see the same pressure. After 200 psi the 'proportion valve" reduces the rear pressure below the front pressure. The higher the total pressure, the greater the difference between front and rear.
A "proportion valve" doesn't care about the "residual valve's" 10 psi, because it operates above 200 psi. The "metering valve" operates from 0 psi to 50 psi, so the 10 psi "residual valve" can upset the way the "metering valve" was intended to operate on the car THAT "combination valve" was intended to be installed into. I've always understood the "residual valve" should be in the "combo valve or after a stand-alone "metering valve" if that is used and one is needed on the front side.
The 50 psi and 200 psi numbers I used are different for every car/truck.
Aftermarket "proportion valves" are adjustable. Many of the aftermarket brake guys also sell separate "metering valves".
The purpose of the "metering valve" is to push the rear shoes against the drum to create some rear drag, before the front brakes begin to work. That is done for stability and to prevent nose dive on your street car/truck. Race cars have stiff suspensions and usually don't need them.
