The one I rebuilt is on the car and working now. It's still in the paint process so no drive time. I found the portion going to the rear was rusted in place not letting any fluid through, top of the picture, the silver shaft with lip seal was stuck. After the cleaning and new seals it was still hard to bleed the system. I had a hold off tool, but it didn't help much. What I did was fabricate a 1/4" thick clear plastic lid with a threaded hole to fit over the master cylinder with a cork gasket. I had a fitting for a vacuum line threaded into the hole. Using my truck as a vacuum source with a long hose, I put the end of the hose up to the fitting but not over it. The vacuum was enough to pull some air out of the rear system, I did it a several times and each time it would pull more air. Too much vacuum will pull air in through the caliper seals, so I was careful with the hose. The vacuum worked so well because the proportioning valve is designed to not let fluid to the rear brakes till the fronts have started to work, by pulling the fluid backwards, there was no restriction and the air came right out. Another '79 owner called about his brakes, so I explained how I bleed them. He did his the same way and in just a few minutes had a solid pedal using the vacuum method.
I don't believe the valve could cause a random brake drag. If both rears or both fronts are dragging, then maybe it could. But if it's just one side, then no.
You need to check for a hot rotor after driving to see if one or more is causing the issue. All new rubber hose is a good idea if they are original or fairly old. A brake booster could also cause a problem if it doesn't return all the way. The push needs some clearance or the master will not return fully which can block the ports and hold the brakes on. Go for the hoses, I've seen them burn up rotors.
Each body style had its own proportioning valve with the correct spring pressure for the car it was designed for. The aftermarket ones fit and work, but may not be 100% correct for your application. Rebuilding them is pretty straight forward once you have the seals. These guys are pretty smart,
https://musclecarresearch.com/kh-kit-1