More and more heavy duty vehicles on the road today are using a Dual Circuit Air System. The system has been developed to shorten stopping distances and reduce brake failures. 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 the rear axle and the other circuit operates the front axle. If one system has a failure, the other system is isolated and will continue to operate.

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.

In the illustration below, air is pumped by the compressor (1) to the supply reservoir (2) which is protected from over pressurization by a safety valve (3). Pressurized air moves from the ‘wet’ supply tank to the primary tank (green) and the secondary tank (red) through one-way check valves (9). At this point, the dual circuits start. Air from the primary tank is directed to the foot valve (6). Air is also directed from the secondary tank to the foot valve. The foot valve is similar to the one described earlier in the single circuit system, but has been divided into two sections (two foot valves in one). One section of this dual foot valve controls the primary circuit and the other section controls the secondary circuit. When a brake application is made, air is drawn from the primary tank (green) through the foot valve and is passed on to the rear brake chambers. At the same time, air is also drawn from the secondary tank (red), passes through the foot valve and is passed on to the front brake chambers. If there is an air loss in either circuit, the other circuit will continue to operate independently. Unless air is lost in both circuits, the vehicle will continue to have braking ability. The primary and secondary circuits are equipped with low pressure warning devices (11) and pressure gauges (10).

SIMPLE DUAL CIRCUIT SYSTEM WITH SPRING PARKING BRAKES

When spring brakes are added to a dual circuit system the same type of dash control valve discussed previously is used. Blended air is used to supply the control valve (19). Blended air is air taken from the primary and secondary circuits through a two way check valve (22). With this piping arrangement the vehicle can have a failure in either circuit without the spring brakes applying automatically. Unless air is lost in both circuits the spring brakes will not apply.

SPRING BRAKES WITH MODULATOR VALVE

Spring type brakes in this system serve two purposes: first, as a parking brake, and second as an emergency system. If a failure occurs in the primary circuit (green), and a brake application is made, control air from the foot valve is directed to a spring brake modulator. As there is no supply air to maintain balance in the modulator valve, due to the primary circuit failure, the modulator valve then exhausts air pressure from the spring brake circuit. The amount of air released is equal to the amount of air applied by the foot valve. The release of air in the spring brake circuit causes the drive axle to brake using spring brake pressure (18). When the brake is released, supply air from the secondary circuit (red) returns the spring brakes to an off position. Brake applications can be repeated until all the air from the secondary circuit is lost but, as the air pressure drops below 85 P.S.I., the spring brakes will not return to the full off position. They will start to drag. At approximately 35 P.S.I., the spring brake control valve (19) on the dash exhaust the remaining air in the secondary circuit, and the spring brakes are fully applied. The only way the vehicle can be moved after all air is lost is to repair the damaged circuit and recharge the system, or cage the spring brake system.

Tandem tractors without steering axle brakes will have the primary and secondary systems split between the drive axles.