Note: All piping diagrams are used to illustrate basic dual circuit principles only, and are not to be interpreted as regulations for, or specifications of, dual air-brake systems.

Virtually all heavy-duty vehicles on the road today are using a dual-circuit air system (Fig. 26). The system has been developed to prevent total brake failures and give you more control by allowing the truck to be brought to a stop in a safe location (Fig. 27). At first glance, the dual system might seem complicated, but if you understand the basic air system described so far, and if the dual system is separated into its basic functions, it becomes quite simple.

As its name suggests, the dual system is two systems or circuits in one. There are different ways of separating the two parts of the system. On a two-axle vehicle, one circuit operates from the primary reservoir and the other circuit operates from the secondary reservoir.

If one circuit has a failure, the other circuit is isolated and will continue to operate.

Under normal operating conditions the primary reservoir operates the rear service brakes and the secondary reservoir operates the front service brakes.