What defines a dual air brake system?

A dual air brake system is characterized by having two separate air brake circuits that work independently while using a common set of brake controls. This design enhances safety and reliability, as it allows one system to take over if the other fails, ensuring the vehicle can still be safely stopped. By having separate circuits, the system can manage pressure differences more effectively and provides additional assurance against brake malfunction.

The other options do not accurately reflect the function of a dual air brake system. For instance, a system with a single air source does not meet the dual system criteria, and requiring manual intervention for each brake contradicts the convenience of a dual air brake system where the driver can apply brakes through unified controls. Additionally, while many air brake systems may operate at higher pressures, the characteristic feature of a dual air brake system is not specifically defined by the PSI levels but rather by its dual-circuit design.