In a pre-trip air brake inspection, which item is checked as part of the air supply circuit?

• A. Park control valve
• B. Tractor protection system
• C. Supply circuit
• D. Air system leaks

Answer: (C)

The main idea is to verify the path that provides compressed air to the braking system, ensuring air is being generated, stored, and delivered at the correct pressures. The air supply circuit is exactly this pathway—from the compressor through the tanks and into the braking system, with the governor regulating the pressure. During a pre-trip, you check that this circuit can build pressure, hold it when the engine is off, and deliver air to the brakes without leaks, so the brakes will function when needed. The park control valve belongs to the parking brake portion, the tractor protection system relates to trailer safety, and leaks are a general concern across the entire air system but not the specific component referred to as the air supply circuit. Therefore, checking the supply circuit is the part of the test that confirms the system’s ability to supply air to the brakes.