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

Question

Accepted Answer

B: Supply air to the trailer system, then pull out the air supply knob.. At this point, it is necessary to supply air to the trailer brake system by pulling out the air supply knob or moving the tractor protection valve control from the emergency to the normal position.

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

Virginia CDL Combination Vehicles Test Info