To prevent all of the air in the tractor from being lost, the trailer air supply valve
- will close automatically only if the vehicle is in motion\
- will close automatically only if the vehicle is stationary
- will close automatically ✓
- must be closed manually
Answer: C. will close automatically
The trailer air supply valve will close automatically.
Why this is the answer
The handbook describes this valve as a push/pull valve that feeds air to the trailer. If you leave it open with no trailer attached, a lot of air can escape from the towing vehicle. To stop that loss, the valve is built to close on its own; it also closes by itself if the trailer breaks loose or air is escaping and supply-line pressure drops. That is why the answer is that it closes automatically, not by driver action or only when moving or stopped.
Why the other answers are wrong
- A. will close automatically only if the vehicle is in motion\ — Automatic closing is not limited to times when the vehicle is moving; the handbook says the valve closes automatically when supply-line pressure drops, such as if air escapes or the trailer breaks loose.
- B. will close automatically only if the vehicle is stationary — It also does not depend on the vehicle being stationary; the valve is designed to close automatically when needed, not only while parked.
- D. must be closed manually — Manual closing is something the driver can do with the red knob, but the handbook says the valve is designed to close automatically to prevent total air loss.
Remember it
Push to feed, pull to close, but remember the valve also shuts itself to save the tractor's air.