Final answer:
Releasing the master controller handle during the brake test engages the emergency brakes, allowing for maximum braking force in case of an emergency.
Step-by-step explanation:
The main purpose of releasing the master controller handle during the brake test is to engage the emergency brakes. When the master controller handle is released, it causes the brakes to apply in emergency, exerting maximum braking force. This step helps ensure that the brakes are in proper working condition and can effectively stop the train or vehicle in case of an emergency.