You are coupling a tractor to the semitrailer; you have connected the airlines. Before backing under the trailer, you should:

Question

Accepted Answer

A: 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 pushing in the air supply knob or moving the tractor protection valve control from the emergency to the normal position.

You are coupling a tractor to the semitrailer; you have connected the airlines. Before backing under the trailer, you should:

Iowa CDL Combination Vehicles Test Info