What happens to the spring parking brakes if air is lost in both circuits?

When air is lost in both circuits of a vehicle equipped with spring parking brakes, the spring parking brakes will apply automatically. This is because the spring brakes are designed to function as a fail-safe mechanism. In normal operation, the air pressure from the braking system is used to hold the springs in a released position, allowing the vehicle to move. However, if there is a loss of air pressure, the springs will be released from their held position and will apply the brakes to stop the vehicle or prevent it from rolling.

This design is critical for safety, ensuring that in the event of an air pressure failure, the vehicle's parking brakes activate and secure the vehicle. This is particularly important on large commercial vehicles where the risk of uncontrolled movement due to brake failure can lead to serious accidents. The spring brakes effectively act as a backup system, ensuring that even in the event of multiple failures in the air delivery system, the vehicle can still be safely immobilized.