Height-Sensing Proportioning Valve ("C" & "K" Series)
WARNING: This page does not describe the selected car, but rather 30 other vehicles, including the 1998 Pontiac Trans Sport, 1998 Oldsmobile Silhouette, 1998 Oldsmobile Bravada, 1998 GMC Yukon, and 1998 GMC Suburban. However, it is still accessible from the selected car via links, so may be relevant.
WARNING:
On vehicles with height sensing proportioning valve, use of aftermarket load leveling kits, air shocks, or modifications that change axle-to-frame distance will provide a false reading to valve. False readings may result in unsatisfactory brake performance.
Valve senses vehicle load conditions through relative movement between rear axle and body. See Figure. As vehicle load increases (resulting in decreased vehicle height), valve allows higher pressure to rear brakes, resulting in optimum front-to-rear brake pressure balance.