A vehicle with air brakes shows a brake warning light when you've just started the engine. What does this mean?

• A. The air reservoirs are fully charged
• B. Increased air pressure
• C. The parking brake isn't working
• D. Low air pressure

Answer: (D)

Air brakes rely on stored compressed air to operate safely. The brake warning light is tied to the system’s low-pressure sensor. If it comes on when you’ve just started the engine, it means the air pressure in the reservoirs is below the safe level. As the engine runs, the compressor fills the tanks and the light should go out once sufficient pressure is reached. If it stays on, there may be a leak, a fault with the compressor or governor, or another issue keeping pressure from building.