Height Sensing Proportioning Valve
General Motors 30 series pickups use a height sensing proportioning valve that responds to changes in vehicle height. This valve automatically provides optimum front-to-rear brake balance, regardless of vehicle load.
The valve controls pressure to rear brakes by sensing vehicle load conditions through relative movement between rear axle and body. As vehicle load increases (resulting in decreased vehicle height), higher brake line pressure to rear brakes is allowed.
WARNING:
The use of aftermarket equipment (such as load leveling kits, air shocks, etc.), or making modifications that change the distance between axle and frame, will provide a false reading to brake proportioning valve. These modifications may result in unsatisfactory brake performance, which in turn could result in an accident and possible personal injury.