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Question:

You are coupling a tractor to a semi-trailer. You have connected the air lines. Before backing under the trailer, you should

A supply air to the trailer system and then pull out the air supply knob to lock the trailer brakes.
Explaination

Before backing under the trailer, check that the trailer brakes are working and then lock the trailer brakes. Once you've attached the air lines, there are a number of checks you should make: Check the glad hand seals. Make sure that the air lines won't be caught or crushed when you back under the trailer. Charge the trailer air brake system and verify that it's working without significant air loss. Then set the trailer air supply control to shut off the air and put on the trailer emergency brakes.