It's fairly easy to determing if a closed-off proportioning valve is causing a condition where no fluid is coming out a rear bleeder valve when attempting to bleed the rear brakes.

Check the amount of pedal that you have when pressing the pedal down before starting to bleed the rears. Then open a rear bleeder valve. If the amount of pedal travel stays the saame, then either: (1)the proportioning valve has slammed to one end, preventing fluid from going to the rear brakes, or (2) the bleeder valve hole is badly clogged and not opening up.

On the other hand, if the pedal goes down further when the rear bleeder valve is open, then it's not a stuck proportioning valve.

If the pedal goes down a lot (like to the floor) with a rear bleeder valve open, and you're not pissing brake fluid out anywhere, then that indicates that you have a lot of air in the rear brake system, and you have to be patient to get it all out. If you have a rear drum brake system, after trying to bleed the rears for a while, take the drums off and be sure you're not pumping fluid through a leaking wheel cylinder.