What is a method to ensure that the trailer brakes are working after supplying air?

Applying and releasing the trailer brakes while listening for brake sounds is a practical method to ensure that the trailer brakes are functioning correctly after air has been supplied. This technique allows the driver to determine if the brakes are engaging and releasing properly. When the brakes are applied, you can hear the sound of the brake system engaging, and when released, the absence of sound indicates that the brakes are disengaging as they should.

This method is not only efficient but also verifies that air has successfully reached the brake chambers and that the pneumatic system is operational. It's crucial for safety that drivers are aware of the condition of their trailer braking systems before proceeding.

Other options, while related to vehicle operation, do not directly confirm the functionality of the trailer braking system. Listening for engine sounds does not provide information on the trailer brakes, inspecting the brake lights focuses on electrical components rather than pneumatic ones, and checking brake fluid levels pertains more to hydraulic systems, which do not apply to trailer brakes that operate on a different mechanism.