A tractor protection valve should be tested by disconnecting the trailer service line and then observing if the valve properly closes to maintain air pressure in the tractor.

To test a tractor protection valve, you disconnect the trailer service line and apply the vehicle brakes. The valve's effectiveness is determined by the stability of the brake system's air pressure; if it remains consistent and doesn't leak out, the valve is functioning correctly.

Explanation:

A tractor protection valve is tested by disconnecting the trailer service line and then applying the vehicle brakes. In normal conditions, you're supposed to hear a popping sound, which is the emergency brakes deploying. The whole point of this test is to ensure that the air pressure is able to maintain within the tractor even when the service line gets disconnected. If the tractor protection valve is working correctly, the air pressure in the brake system should remain stable and not leak out. Therefore, this test is very important as it ensures the safe operation of the tractor-trailer.

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