If air pressure is insufficient to hold back the spring brakes, which brake component may engage to slow the vehicle?

• A. Front brakes
• B. Engine brake
• C. Air compressor
• D. Spring brakes

Answer: (D)

Spring brakes are designed to apply when air pressure is no longer enough to keep them released. In an air-brake system, the springs are held back by air pressure; if that pressure drops, the springs snap into braking position, slowing and stopping the vehicle as a safety measure. The front brakes (service brakes) require adequate air pressure to function, and the engine brake operates through engine mechanics rather than the spring mechanism. The air compressor furnishes the pressure but doesn’t apply braking itself. So when pressure can’t hold the springs back, the spring brakes engage to slow the vehicle.