disc brake pre72/post73 proportioning valves

The P valves just supply full pressure to the front 2 brake lines (common plenum inside) and then there is a spring loaded piston that is preloaded to like 300 lbs that gets forced back when you apply the brakes that uncovers the rear feed. Its a way to apply the fronts prior to the rears. It also serves as a brake warning light. I bet most if not all do the same thing and are biased equally.