Before starting to back a combination vehicle, which of the following checks is most important?

Ensuring the air supply is connected is crucial before starting to back a combination vehicle because the braking system relies on air pressure to function properly. In combination vehicles, trailers often use air brakes, which need a functional air supply system for effective stopping power and overall safe operation. If the air supply is not connected, the brakes may not engage appropriately, leading to significant safety hazards while maneuvering the vehicle.

While verifying the tractor's fuel level, checking the alignment of the trailer, and examining the tires for wear are all important safety checks, their direct impact on the immediate task of backing up the vehicle is less critical than confirming that the air supply is properly connected to ensure the braking system operates effectively. Properly connected air supply systems enable the driver to have full control over the vehicle’s stopping and maneuvering capabilities, thereby preventing potential accidents or loss of control.