A spring brake control valve is normally a push/pull type valve fitted with a yellow, four-sided knob located near the driver. Most spring brake control valves are pushed to supply air and release the spring brakes, then pulled to exhaust air and apply the spring brakes. Some vehicles may have this function reversed, but its functions are normally described on or near the control valve. Some vehicles use a toggle type valve for this purpose. Drivers must be familiar with the type of control valve used in their vehicle.
Some trucks and tractors may also have a separate control called a tractor parking brake control valve to release the spring brakes on the tractor while keeping the trailer spring brakes applied. This optional control valve normally has a round blue knob.
Spring brake control valves are designed to respond to air brake system pressure dropping below a certain level (normally 414 kPa or 60 psi) by exhausting the remaining air that is holding the spring brakes in the released position. This causes sudden automatic application of the spring brakes and an uncontrolled vehicle stop. The control valve knob will pop out when this occurs.
Important: If air brake system pressure drops below its normal operating range (normally 414 kPa or 60 psi), the spring brakes will automatically begin to apply.
In an emergency when the service brakes fail, the spring brakes can be applied by using the spring brake control valve.
The effectiveness of a vehicle’s spring brakes depends on the condition of the brakes and proper brake adjustment. If brakes are out of adjustment, the spring brakes may not stop or hold a vehicle stationary.
Remember: Poor brake adjustment reduces the ability of service brakes to stop a vehicle and reduces the ability of spring brakes to stop or hold a vehicle.