The proportioning valve is pretty simple, I wouldn't think it would get stuck unless the lines have been dry for a while.
At the prop valve, loosen the line going to the rear brakes just a little and have someone push on the brakes. Basically bleed the brakes at the valve. If fluid comes out it's time to move to the back wheels.
If you haven't replaced the rubber brake line at the rear of the car I would suspect thats your problem (it's not at the wheel cylinder, follow the metal lines at the rr wheels to the junction block on the axle). You have to have a rubber line there cause the axle moves independently of the car body.
Loosen the downstream end of the rubber line and see if you can get fluid out of it.
Before you start all this spray some penetrating oil on everything and let it sit as long as you can, makes things much easier, those old brake lines are real easy to twist up if the nut won't spin on them.