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After connecting the air lines but before backing under the trailer, you should

A supply air to the trailer system and then pull out the air supply knob.
Explaination
First, make sure that the trailer brakes are working properly. Before backing under the trailer, set the trailer air supply control to lock the trailer brakes. You do this by pulling out the red air supply knob (on newer vehicles) or moving the tractor protection valve control to the emergency setting.
B walk around the rig to make sure it is clear.
C make sure that the trailer brakes are off.