The Trailer Air Supply control (red octagon knob):

Supplies air to the trailer tanks and controls the trailer emergency brakes.. Section 6.2: You push it in to supply the trailer with air,

The Trailer Air Supply control (red octagon knob):

ANSWER BREAKDOWN
  • A. Incorrect. "Controls the radio." is not correct. You push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes.
  • B. Correct. You push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes.
  • C. Incorrect. "Applies the service brakes." is not correct. You push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes.
  • D. Incorrect. "Releases the tractor parking brake." is not correct. You push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes.