Components of the spring-brake trailer air system (Fig. 49) are:

• front-service reservoir
• rear-service reservoir
• trailer spring-brake valve
• relay valve (same as on tractor - not an emergency relay valve as used on trailers)
• spring-brake chambers

The new component - the trailer spring-brake valve (Fig. 50) - is responsible for several important functions:

• It controls application and release of the trailer’s spring brakes.
• It protects and isolates the front-service reservoir from the rear-service reservoir. This is an important feature that prevents an automatic application of the spring-brakes, even though the trailer’s service reservoir is lost.
• It prevents automatic spring-brake application if the trailer’s supply line has a gradual leak.
• It will automatically apply the spring brakes if supply pressure is rapidly lost (after a breakaway).

You can check the operation of the trailer spring-brake valve by closing the supply valve on the tractor or by disconnecting the supply line between the tractor and trailer with the supply valve in the open position.