Inversion Valve
Straight trucks and some tractors use an Inversion Valve which does not function during normal operation of the vehicle’s brake system but operates in the event of a primary (rear service brake) system failure. When the Inversion Valve senses an imbalance of the primary and secondary delivery pressure during an application of the foot valve, it inverts that pressure to release air from the spring brake chambers. This permits the modulated application of the mechanical spring brake chambers on the rear axle(s) to assist the front brakes in stopping the vehicle safely.
Inversion Valve Catalog Page