Answer :
Final answer:
To test the trailer service brakes, ensure the parking brake is set and press the brake pedal while listening for air leaks. The correct steps are not fully listed in the options, and the reference provided is not directly related to trailer service brakes.
Explanation:
The correct procedure for testing the trailer service brakes only is not explicitly listed among the provided options. However, the proper method typically involves first ensuring the parking brake is set to prevent the vehicle from moving. Then, with the engine running to maintain air pressure, you would press the brake pedal. As you apply pressure to the brake pedal, you listen for any air leaks, which can indicate issues with the brake system. The options provided mix elements of different tests or are steps for other procedures. For instance, pulling the trailer air supply control knob or pushing in the tractor protection valve control are operations related to the emergency brake system, not the service brake test.
It's important to follow your vehicle's specific guidelines, which may differ from generic procedures. Always reference the vehicle's manual and any training provided to perform brake tests correctly. The provided reference to closing a stopcock on an evaporator and the hissing sound stopping is an example of how bleed valves work but does not directly correlate to the air valves operation in the context of trailer service brake tests.
