Which adjustment should be made if the trailer's brakes do not engage properly?

Adjusting the trailer air supply control is the correct answer because this control is crucial for ensuring that air pressure is adequately supplied to the trailer's braking system. When the trailer's brakes do not engage properly, it often indicates that there is insufficient air pressure reaching the trailer's brake chambers. By adjusting the air supply control, you can enhance the flow of air to the brakes, allowing them to function correctly.

Ensuring that the trailer receives the appropriate amount of air pressure is essential for the timely engagement of the brakes, thus maintaining safe operation. Improper adjustment or malfunction of the air supply control can directly impact the performance of the trailer’s braking system and is a key area to inspect if braking issues arise.

Other actions such as releasing the spring brake, checking the compressor functionality, or draining air tanks do not specifically address the air supply to the trailer’s brakes. While those actions might be relevant under different scenarios, they do not resolve the immediate concern of inadequate engagement of the trailer brakes caused by insufficient or misdirected air supply.