In an air-brake system, what factor ensures the spring brakes remain retracted during normal operation?

Answer: B — Air pressure

Spring brakes are spring-applied, air-released. The air pressure in the brake chambers counteracts the force of the release springs, keeping them retracted and the brakes released during normal operation. If air pressure drops, the springs push out to apply the brakes. Hydraulic pressure, oil temperature, or tire pressure don’t control this release mechanism.