A dual air brake system is a braking system that consists of two separate air brake systems, with their own sets of hoses, lines and tanks that use the same set of brake controls. Often, dual air brake systems correspond to different axles on large vehicles. In this type of setup, one set of equipment is called the primary system, and the other setup is the secondary system.
Air brakes are actually created using three different types of braking systems: service brakes, parking brakes and emergency brakes. In an air brake design, compressed air is used on all of these brakes to transfer pressure and ultimately give the vehicle its stopping power. Because of the amount of compressed air used, air braking systems require their own air tanks and must be maintained regularly to make sure they still function well.
Heavy-duty vehicles, like buses and trucks, often have two different air braking systems. Because they are so large, the use of a dual air brake system allows for the pressure transmission needed to stop these large vehicles to spread efficiently, according to the California DMV. Usually, each braking system corresponds to a different set of front or rear axles. These types of air braking systems need to build up some pressure before they are able to function.