What should you ensure before backing a tractor under a trailer after pushing the trailer air supply valve?

Before backing a tractor under a trailer after pushing the trailer air supply valve, it is crucial to ensure that the air system is at normal pressure. This is important because the air braking system in combination vehicles relies on adequate air pressure to function properly. If the air system is not at normal pressure, it can lead to insufficient brake function, which poses a significant safety risk while coupling the tractor and trailer.

Maintaining normal pressure ensures that the service brakes and the trailer's brake system are fully operational when the connection is made. It also indicates that the trailer air supply has been successfully engaged, which is essential for the safe operation of the vehicle. Other factors, while important in their own right, do not directly relate to the necessary air supply for proper braking and coupling procedures. For instance, the security of the service line, the condition of parking brakes, and the level of the trailer are certainly relevant while handling trailers but they are secondary to confirming that the air pressure is adequate for safe operation.