proportioning valve questions

Post #4, but your drum one is termed "distribution block". Nothing inside, just a "tee". You can use it, but add an adjustable proportioning valve in-line w/ the rear tube. That is what the factory did in the early disk cars (until ~1969). Or replace the block with a "combination valve" (post #10).

"Locking up" implies the rears don't release with the pedal. That could be from a loose flap of rubber in a hose, acting like a check valve, or rust gunk sitting on a port. Post #6 appears to answer a different issue (rears too forceful).

Drum MC's often had a "residual valve" in each port, which is a check valve. The intent was to hold slight positive pressure (~10 psi) in the wheel cylinders, but not enough to overwhelm the springs. That kept the rubber cups tight so air didn't get past them. I understand they weren't needed after they went to the "springs w/ cups" or "springs with tightly wound ends" maybe 30 years ago. All "wheel kits" today have those. Leaving residual valves in has the opposite effect. They cause front disk calipers to lock up.