CDL Vehicle Combination Practice Test

To supply air to the trailer tanks, you push the trailer air supply valve. This action releases the valve, allowing compressed air from the tractor's air system to flow into the trailer's air tanks. This is essential for the trailer's braking system, which operates on air pressure. Properly supplying the trailer with air ensures that the brakes can engage and function effectively when needed. Pushing the trailer air supply valve is the correct action to initiate this process. In contrast, other options do not directly accomplish the goal of supplying air to the trailer. For example, pulling the valve may not engage the air supply, and merely releasing the parking brake does not influence the air supply to the trailer. Lastly, turning the ignition on is necessary for the operation of other electrical components but does not directly relate to supplying air to the trailer.