What is essential for the driver to perform under emergency circumstances regarding the brake system?

The essential action for the driver to perform under emergency circumstances regarding the brake system is to engage the park brake switch. Engaging the park brake is a critical safety measure that can help bring a vehicle to a stop safely when the primary braking system fails. This action provides a secure means of holding the vehicle in place, preventing it from rolling or moving unexpectedly, which is especially important in emergency scenarios.

While using the horn, activating the auxiliary braking system, or notifying the control center can be important actions during an emergency situation, they do not directly address the immediate need to control the vehicle's motion when the standard braking system is compromised. The primary focus in such a scenario is ensuring the driver's ability to stop or stabilize the vehicle, making the engagement of the park brake the most crucial response.