What should you hook up before backing under a semi trailer?

Hooking up the emergency and service air lines before backing under a semi-trailer is crucial for ensuring that the trailer's brakes function properly once the trailer is connected to the tractor. The emergency air line is responsible for activating the trailer's emergency braking system in case of disconnection while on the road, while the service air line controls the regular brakes when you apply the brake pedal in the tractor. This is a fundamental safety procedure in securing and operating a semi-truck and trailer combination, as it directly impacts the ability to stop the trailer and maintain control.

Other components like the trailer hitch and safety chain are important, but they typically come into play after establishing a connection with the air lines. The backup camera, while useful for visibility and aiding in the reversing process, does not directly relate to the safety and operational functionality of the semi-trailer once it is coupled. Hence, ensuring the air lines are connected first is a priority in the hooking up process.