Scott,
I forgot to answer your question. The prop valve is a crutch for a system that isn't sized properly. Manufacturers don't want to design brand new components for every vehicle. It's cost prohibitive. A prop valve reduces line pressure to the output line of the valve. It's typically done to reduce the amount of rear brake.
The WRX has two (in the same package). They allow line pressure to build equally until a given pressure is reached. Once that pressure is reached, the output pressure "knees" to a less steep ramp as input pressure is increased. Essentially it will allow the rear brakes to do more work at light brake application, but ensures that the fronts do more work as brake pressure is increased, keeping the car safe.
Gary
Sheehan Motor Racing
www.teamSMR.com