Proportioning Valve
WARNING: This page is about a different car, the 2005 Jeep Liberty. However, it is still accessible from the selected car via links, so may be relevant.
The proportioning valve is used to balance front-rear brake action at high decelerations. The valve allows normal fluid flow during moderate braking. The valve only controls fluid flow during high decelerations brake stops. If the primary brake hydraulic circuit cannot build pressure a by-pass feature is activated allowing full flow and pressure to the rear brakes.