Help me understand the brake proportioning valve

Is it possible you have a collapsed hose on either side on the front?

I had a dodge once that wore through rear brakes a lot. Thought it was something wrong with the rear brakes. Turns out the front rotors / pads were really glazed. The rear brakes were doing all the work. Replaced rotors and pads and problem went away. Car stopped better too.

The moral to the story is that if it is not what you think it is, it might be something else causing the problem indirectly.