If the brakes are not released after pushing the trailer air supply valve, what should you check?

The correct response emphasizes the importance of the air line connection in a combination vehicle's braking system. When the trailer air supply valve is pushed, it should release the brakes on the trailer if everything is functioning correctly. If the brakes do not release, it indicates there may be an issue with the air supply reaching the trailer's braking mechanism.

A malfunction in the air line connection, such as a leak or obstruction, would prevent the necessary compressed air from reaching the trailer's brakes, which leads to them remaining engaged. Therefore, checking the air line connection is essential to ensure that the system is operating properly and to troubleshoot any potential brake release issues.

The other options focus on aspects that are less relevant in this specific scenario. For instance, brake fluid levels are not applicable to trailer brakes, which typically operate on air pressure rather than hydraulic fluid. The coupling mechanism, while important for connecting the trailer to the tractor, does not directly affect the air brake functionality. Tire pressure is crucial for overall vehicle safety and performance but does not play a role in whether the trailer brakes release when commanded by the air supply valve.