Answer :
Final answer:
To test the trailer service brakes, the correct procedure is to pull out the red "tractor protection valve" button and apply the trailer handbrake. Doing so ensures that the brakes on the trailer are tested independently of the tractor for safety and functionality.
Explanation:
The correct answer for the air valves when testing the trailer service brakes only is B. Pull out the red "tractor protection valve" button and apply the trailer handbrake. The tractor protection valve is typically a red eight-sided knob found on the dashboard of the tractor. When you pull this valve out, it cuts the air supply from the tractor to the trailer and thus the trailer brakes are applied separately when you engage the trailer handbrake. This is part of a standard procedure for performing a trailer service brake check to ensure safety and proper functioning of the braking system. It's essential that drivers understand this process to prevent accidents and maintain effective control of the trailer.
