What is the standard release requirement for the parking brake system during a failure?

The standard release requirement for the parking brake system during a failure entails that the brake should be able to release at least once when either the primary or secondary system experiences a pressure failure. This requirement is crucial because it ensures that, in the event of a failure, the air brake system can still operate effectively to release the parking brake. It allows for a safety measure that can help prevent potential accidents or loss of control of the vehicle.

Other options lack the necessary context or importance of the situation. Releasing the parking brake immediately after any failure may not account for whether the failure is manageable or if the vehicle is still in a controlled state. Releasing the brake only during scheduled maintenance checks does not address real-time operational safety. Lastly, releasing the parking brake only when the vehicle reaches a complete stop does not provide the immediate safety response required during specific failure conditions, making it ineffective in critical situations. Therefore, the correct approach emphasizes the need for a reliable release during a failure event.