What must the air system be at before backing a tractor under a trailer?

The air system must be at normal pressure before backing a tractor under a trailer because this ensures that the braking and suspension systems are fully operational. When the air system is at normal pressure, the air tanks contain enough compressed air to support the vehicle’s braking system, preventing potential malfunctions.

Backing the tractor under a trailer while the air system is below normal pressure could lead to inadequate braking power or a failure to properly engage the trailer's brakes once connected. This situation increases the risk of accidents or uncontrolled movement. Therefore, ensuring the air system is at normal pressure is critical for safety and operational reliability during the coupling process with the trailer.