Back Brakes won't bleed

For many cars, that is not a "proportioning valve". It is a pressure differential sensor that activates the brake warning light on the dash if there is an imbalance between front and rear brake line pressure. This usually indicates a problem with the master cylinder or a leak somewhere. This device nearly never fails and I would be highly surprised if yours is bad.

If your rear brakes are not bleeding, then you have a clogged line somewhere. Start by disconnecting the flex hose to the rear diff and have someone step on brake pedal. If no fluid comes out, then the clog is closer to the master cylinder. Keep disconnecting lines until you see fluid coming out when the brakes are applied. Then clear or replace that line(s).