You are coupling a tractor to a semi-trailer. You have backed up but are not yet under it. Before backing under the semi-trailer, you should hook up
the emergency and service airlines.
Before backing under the trailer, you must lock the trailer brakes – which means that you first have to make sure that the trailer brakes are working. Connect the emergency and service airlines and test the trailer brakes.
Take more free practice tests for other CDL topics with our cdl tests