How should you check the functionality of the trailer emergency brakes?

To effectively check the functionality of the trailer emergency brakes, pulling out the trailer air supply knob and gently pulling on the trailer is the correct approach. This process ensures that the emergency brakes are engaged and operating properly.

When the trailer air supply knob is pulled out, it allows the trailer’s air supply to activate. By gently pulling on the trailer—typically done by a tow vehicle or manually—you can observe if the brakes engage and hold the trailer in place. This test is crucial to ensure that the emergency brakes function properly in case of a situation where they are needed, such as a loss of air pressure in the system or if the trailer becomes detached.

Performing this action checks the brake response and effectiveness, confirming their operational integrity before heading out on the road. This step is vital for safety and compliance with vehicle regulations.

The other options do not adequately test the emergency brakes' functionality. For instance, releasing the parking brakes or simply driving forward does not confirm whether the emergency brakes engage correctly. Examining the brakes while stationary may provide some insights into their condition but fails to demonstrate their operational capability in a dynamic context. Therefore, the chosen method best fulfills the requirement to ensure that the emergency brakes are ready for use.