What does an air brake system use to release air pressure when brakes are not applied?

Answer: B — spring brake mechanism

An air brake system utilizes a spring brake mechanism to release air pressure when the brakes are not applied. This system is designed to engage the brakes automatically in the event of air pressure loss, ensuring that the vehicle comes to a stop safely. The spring brakes are held in the released position by air pressure; when that pressure is removed, the springs engage and apply the brakes, effectively stopping the vehicle. This mechanism is crucial for maintaining safety, providing a reliable method to prevent runaway vehicles. It highlights the importance of air pressure in managing both the application and release of the brakes, showcasing how braking systems are designed to fail-safe in case of issues with air supply.