Most vehicles equipped with air brakes use a dual brake system, also known as a dual air brake system. This system is safer because it allows for a secondary course of action in the event that the primary braking system fails.
The purpose of a dual air brake system is to accommodate a mechanically secured parking brake that can be utilized during a service brake failure and to accommodate the need for a modulated braking system in case either or both systems fail.
The dual air brake system is actually two brakes in one, giving the system a larger reserve capacity. On a two-axle vehicle, like a sedan, one circuit focuses on the front axle while the other operates the rear; if one circuit fails, the other is isolated and remains active, allowing the vehicle still to utilize its brakes.
When the brakes are applied, air is taken from the primary reservoir and delivered to the rear brakes; simultaneously, air is taken from the secondary reservoir and delivered to the front brakes. If a leak develops in either system, the other continues to operate independently, allowing the vehicle to retain its braking ability.
