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

Supply air to the trailer system, then pull out the air supply knob to lock the trailer brakes.. Section 6.4: You want the trailer brakes

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

ANSWER BREAKDOWN
  • A. Correct. You want the trailer brakes locked so the trailer doesn't move when you back under it.
  • B. Incorrect. "Disconnect the lines." is not correct. You want the trailer brakes locked so the trailer doesn't move when you back under it.
  • C. Incorrect. "Drive away." is not correct. You want the trailer brakes locked so the trailer doesn't move when you back under it.
  • D. Incorrect. "Check the oil." is not correct. You want the trailer brakes locked so the trailer doesn't move when you back under it.