In a dual air brake system, the design ensures that there are two separate circuits for the brakes. This redundancy is crucial for safety because it allows the vehicle to maintain braking ability even if one circuit fails. If one circuit encounters a malfunction, the other circuit can still function, providing sufficient braking force to stop the vehicle safely. This dual system is specifically implemented in larger vehicles and trucks to prevent complete brake failure, ensuring that the driver still has control and can bring the vehicle to a stop, albeit possibly with some operational limitations depending on the situation.
In contrast, the other options imply complete or significant loss of braking ability, which goes against the fundamental safety design of a dual air brake system. Therefore, the vehicle's continued braking ability with one circuit failure is a critical safety feature incorporated into modern braking systems.