Proportioning Valves (Montana, Silhouette & Venture)
WARNING: This page does not describe the selected car, but rather 46 other vehicles, including the 2001 Pontiac Montana, 2001 Pontiac Aztek, 2001 Oldsmobile Silhouette, 2001 Oldsmobile Bravada, and 2001 GMC Sonoma. However, it is still accessible from the selected car via links, so may be relevant.
System uses 2 proportioning valves, threaded into master cylinder, to regulate brake system hydraulic pressure.
Valves allow input pressure to rise to predetermined level before allowing output pressure to rear brakes. This prevents rear wheel lock-up on vehicles with light rear wheel loads. Valves also incorporate a by-pass feature which ensures full system pressure is applied to rear brakes if front brakes lose hydraulic pressure (or if rear brakes lose hydraulic pressure, full pressure is applied to front brakes).