Answer :
Final answer:
Protection valves in towing vehicles prevent air loss from the trailer emergency line to ensure that the vehicle can still be controlled in the event of an emergency, such as a significant drop in air pressure.
Explanation:
Protection valves in towing vehicles are critical components that ensure the safety of the vehicle's air brake system. These valves are designed to prevent air loss from the trailer emergency line, which is the correct answer to the question. The emergency line is responsible for activating the trailer brakes in case of a significant drop in air pressure, thereby preventing a runaway trailer situation. It is also used to apply the trailer brakes in an emergency stop.
When the air pressure in the emergency line drops below a certain level, the protection valve closes, preserving air in the tractor's system so the driver can still control the vehicle. Since the tractor service line, trailer service line, and parking brake line are part of the overall air brake system but do not directly pertain to the emergency functions that the protection valves are designed for, they are not the correct answers in this context.
