Which system must be at norm before backing the tractor under the trailer?

Before backing the tractor under the trailer, it is crucial that the whole air system is at norm. This is because the air system is responsible for operating critical functions such as the trailer brakes and the air suspension. If the air system is not functioning properly, it could lead to inadequate braking or instability while maneuvering, which can compromise safety during the coupling process. Ensuring that the air system is fully pressurized and operational helps maintain control of the vehicle and ensures that the trailer can be safely secured to the tractor.

While other systems like the fuel, electrical, or hydraulic systems are also important for the overall operation of the vehicle, they do not directly influence the immediate safety and functionality of connecting the tractor to the trailer in the same way that the air system does.