Answer :
Final answer:
Upon a trailer breakaway, the trailer brakes are expected to engage due to loss of air pressure. The tractor protection valve will close to safeguard the tractor's air supply, but the trailer supply valve becomes ineffective once the trailer is no longer attached. The parking brake would not engage automatically but can be applied by the driver for additional control.
Explanation:
When a trailer breaks away in a combination vehicle, causing both air lines to pull apart, several things are expected to happen. The trailer brakes are designed to automatically engage in such a scenario to prevent the trailer from moving uncontrollably. This safety feature is due to the emergency line carrying air to fill the trailer's air tanks, and once disconnected, the air pressure drops, triggering the springs in the brake chambers to apply the brakes.
Additionally, you would expect the tractor protection valve to close. This protects the air supply in the tractor from depleting rapidly, which allows the tractor's brakes to continue functioning properly. The valve senses the loss of pressure from the trailer breaking away and shuts off automatically. The trailer supply valve, also known as the emergency valve, is intended to stay open to provide air to the trailer.
However, when a separation occurs, this valve effectively becomes useless since there is no longer a trailer to supply.
Lastly, the system parking brake in the tractor would not close just from the trailer breaking away, assuming it was not engaged before the incident. The parking brake is controlled independently by the driver, and while it may be a good idea to engage it in this situation to control the vehicle, it would not engage automatically as a direct result of the trailer disconnecting.
