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Daily inspection test
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4. Inoperative towing vehicle (tractor) protection valve

You must know the proper method for testing the tractor (towing vehicle) protections system, and that a vehicle has this major defect when air escapes from the service line during service-brake application, when the trailer supply valve is closed.


    • Ensure air pressure is within its normal operating pressure range, cut-in and cut-out values (80 -135 psi).
    • Ensure the trailer supply valve is closed (pulled out).
    • Remove the trailer service-line coupler (glad-hand) from the trailer or its storage location, and place it where it can be observed.
    • Apply service brakes.
      Note: If you are concerned that the vehicle has no anti-compounding valve, you must ensure the vehicle is secure and release the spring (parking/ emergency) brakes before applying the service brakes.
    • Observe if any air is exhausting from the trailer service-line coupler (glad-hand).