When coupling a tractor to a semi-trailer, you have linked the air lines. Before backing under the trailer, what should you do?

Confirm the trailer brakes are operational before backing beneath the trailer, and then lock the trailer brakes. Following certain checks is necessary before installing the air lines: Look at the seals of the gladhand. Make sure the air lines are not hooked or crushed as you reposition the vehicle beneath the trailer. Charge the air brake system on the trailer and check to see if there isn't a lot of air loss. Apply the trailer emergency brakes after that and turn off the trailer air supply control.