What is the last action performed upon successful completion of the air brake test?

The last action performed upon successful completion of the air brake test is to fully apply the independent brakes. This step is crucial because it ensures that the independent brakes are functioning correctly and provides a final confirmation that the system is operational before proceeding with any further movements. By fully applying the independent brakes, the train operator can assess the effectiveness of the braking system and ensure that the train can be safely stopped if necessary. This step helps in verifying the integrity of the air brake system, confirming that the brakes can hold the train in position, which is vital for operational safety.

While other actions mentioned are part of the air brake test process, they are not the last steps and serve different purposes, such as checking functionality or preparing the train for movement.