Do Proportioning Valves Wear Out?

My 67 GTS was not correctly engineered from the factory and it had premature rear wheel lock up. Years ago I got the Mopar Performance adjustable valve. I had to set it to the maximum pressure reduction to the rear brakes.

A few years ago when I replaced the scored front rotors I put on slotted rotors on and ferrisgraphite pads. Now the car stops the best it ever has. Ferrisgraphite pads are suppose to have 25% more stopping force and are rotor friendly.