Pressure Reducing Valve
WARNING: This page is about a different car, the 1999 Land Rover Discovery. However, it is still accessible from the selected car via links, so may be relevant.
Pressure Reducing Valve (PRV) is used to maintain balanced braking by regulating hydraulic pressure to the rear brakes. If there is a failure to the front brake circuit, full system pressure will be routed to the rear brakes.