Before coupling a second trailer, you should release the dolly brakes by:

Opening the air tank petcock.. Section 7.2: On some dollies, you open the air tank petcock to release the brakes so you can move it by hand.

Before coupling a second trailer, you should release the dolly brakes by:

ANSWER BREAKDOWN
  • A. Correct. On some dollies, you open the air tank petcock to release the brakes so you can move it by hand.
  • B. Incorrect. "Pushing in the air supply knob." is not correct. On some dollies, you open the air tank petcock to release the brakes so you can move it by hand.
  • C. Incorrect. "Draining the air tank." is not correct. On some dollies, you open the air tank petcock to release the brakes so you can move it by hand.
  • D. Incorrect. "You cannot release them." is not correct. On some dollies, you open the air tank petcock to release the brakes so you can move it by hand.