On an air-braked vehicle, if the brake warning light is on just after starting the engine, what does it indicate?

• A. The air reservoirs are full
• B. The brakes require adjustment
• C. The parking brake is applied
• D. Low air pressure

Answer: (D)

The brake warning light on an air-braked vehicle is tied to the air pressure in the system. The braking system relies on stored compressed air, and the warning light comes on when that pressure falls below the safe minimum. If it lights up just after starting the engine, it means the air pressure hasn’t built up to the required level yet (or there’s a loss that needs attention). The light is designed to alert you before moving so you don’t rely on brakes that might not respond properly. It should go out once the air reservoirs reach the normal operating pressure. If it stays on, it indicates low air pressure and you should only proceed after the system has built up adequate pressure or you investigate the cause.