What is the basic principle of a dual air brake system?

Answer: B — mechanically-secured parking brake that applies in service brake failure

The basic principle of a dual air brake system focuses on the redundancy and safety provided by having two separate air brake circuits. When one system experiences a failure, the other remains operational, allowing for continued braking capability. The presence of a mechanically-secured parking brake serves as an additional safety feature that automatically activates in the event of a service brake failure. This ensures that the vehicle can still be brought to a stop safely, even if one of the circuits malfunctions. Understanding this principle highlights the importance of safety in vehicle braking systems, as it protects against potential total brake failure, which could lead to accidents. The use of a dual setup is especially crucial in larger vehicles, where the consequences of brake failure would be more severe.