CDL Vehicle Combination Practice Test

What must be done first to supply air to the trailer?

1. Turn off the parking brake

2. Stop engine so you can hear the brakes

3. Attach the glad hands

4. Check the air pressure gauge

The correct answer is: Stop engine so you can hear the brakes

The first step to supply air to the trailer involves ensuring that the air system is set up correctly; stopping the engine to hear the brakes is a critical part of this process. When the engine is stopped, you can listen for any air leaks and confirm that the brakes are functioning properly. This is an important safety check before you proceed, as it allows you to ensure that the trailer's air supply system is intact and ready. While attaching the glad hands is essential and necessary to connect the trailer to the truck's air supply system, it should only be done when you are sure that there are no air leaks and that the air pressure is adequate. Checking the air pressure gauge is also important, as it provides information about the system's readiness, but it is typically done after ensuring that the environment is safe (including the state of the brake system). Turning off the parking brake is generally a later step that cannot be performed without addressing the more critical safety checks first. In summary, stopping the engine gives you the opportunity to conduct crucial safety checks, making it the most appropriate initial step to supply air to the trailer.