I have a 70 Camaro front disk/back drum. Last month I drove the car about 20 miles on a cold evening and as I pulled into the drive and I noticed steam/smoke from the rear wheels. I had no idea they had been dragging but they were so hot I literally thought the car was on fire. Anyway, there are no leaks and I did re-bleed but that was not the issue. I still have the same issue where the drums drag. Is this a proportioning valve issue?? I believe it is but not sure. Im trying to find the exact same prop valve however I am having such a hard time finding a bottom mount valve that has one external for passenger and two for driver front and driver rear?? I cannot for the life of me find one like this. I also ended up changing all the guts of the drums...pads/springs/wheel cylinders.
All of the valves I see online are set up with the lines coming out of the front for the passenger wheel, the bottom external for the driver front wheel and for the rear its directly out the back. This version would require I bend all the existing brake lines and that to me would be a pain in the ***.
As it is Ive spent enough cash on a booster/cylinder/valve and so far through process of elimination Ive only changed out the booster(figured I would do the hardest thing first to see if that was indeed the problem) So on to the cylinder and valve.

Any advice?

 

I had a similar issue and the plunger in the bottom of my master cylinder had worn a groove through so it was allowing the 10 psi residual from the front bleed into the back brake lines that are supposed to be at 3 psi residual and were equalling out about 6 psi so the rear brakes would drag took me for ever to figure that one out

 

I had a drum drag on mine and cleared it up by simply spraying liberally with Brakleen.

 

If it's both rear drums, then the problem most likely is ahead of the drums. Might be a master cylinder issue, and since they are the easiest/cheapest to replace, I'd start there. About $18 at the chain stores exchange.