Which part of the air brake system is described as keeping air in the tractor or truck brake system?

• A. The air dryer
• B. The advisory gauge
• C. The tractor protection valve
• D. The brake pedal

Answer: (C)

Keeping air pressure available for the tractor’s own braking system is the job of the tractor protection valve. This valve sits in the supply line to the trailer and is designed to isolate the tractor’s air system from the trailer if the trailer becomes disconnected or a significant leak occurs. When everything is connected and operating normally, the valve stays open, allowing air to flow to both the tractor and trailer brakes. If the trailer line loses pressure or detaches, the valve closes and traps air in the tractor’s own lines and reservoirs, so the tractor still has enough pressure to stop. The air dryer removes moisture from the air, the advisory gauge simply shows pressure, and the brake pedal activates the brakes but doesn’t isolate or preserve air in the tractor’s system.