2.1 Dual Air System Circuits

Dual Air System Circuits

In heavy commercial transport, vehicle weight, cargo loads, and demanding operating conditions necessitate a braking system of absolute reliability. Historically, older commercial vehicles utilized a single air brake system. In that layout, a single ruptured hose, failing seal, or loose fitting could exhaust the entire system's air supply, leaving the operator with zero service braking capacity. To eliminate this catastrophic vulnerability, modern commercial vehicles are built with a dual air brake system. This layout is now mandatory for on-highway commercial operations in British Columbia under the Insurance Corporation of British Columbia (ICBC) and provincial motor vehicle regulations.

A dual air brake system is defined by its redundancy. Rather than operating as one large circuit, it splits the service braking system into two distinct, independent circuits: the primary circuit and the secondary circuit. Each circuit possesses its own dedicated air reservoirs, independent plumbing (hoses, lines, and valves), and brake chambers. While both circuits share the same air compressor and governor as their common charging source, they are isolated from one another down-system. This division ensures that a failure, leak, or pressure drop in one circuit does not compromise the operational integrity of the other.

The Division of Circuits: Primary and Secondary

The primary and secondary circuits are assigned to different axles to ensure balanced braking control during an emergency. In a standard two-axle truck or tractor:

• The Primary Circuit typically operates the brakes on the rear axle or axles. On a commercial vehicle, the rear axles carry the majority of the vehicle's payload and perform the bulk of the braking work under normal operating conditions. Therefore, the primary circuit is engineered to handle these heavy-demand chambers.
• The Secondary Circuit typically operates the brakes on the front (steering) axle. Front brakes are critical for steering control and stability, and their air supply must be kept independent of the rear brakes. On some multi-axle configurations or buses, the secondary circuit may also supply air to auxiliary axles or specific safety components, but its fundamental role remains the management of the front steering axle's service brakes.

This division ensures that if the primary circuit fails, the front steering axle brakes remain functional. Conversely, if the secondary circuit experiences a total leak, the rear drive axle brakes continue to operate. While stopping distance is affected when only one circuit is active, the vehicle does not experience a total loss of service brakes.

Summary of Circuit Component Layout

The Dual Foot Valve (Treadle Valve)

Because the dual air system consists of two independent circuits, a mechanism is required to apply both circuits simultaneously and proportionally. This is the function of the dual foot valve, also referred to as the treadle valve or split treadle valve.

Located under the driver's brake pedal, the dual foot valve houses two separate, independent valves within a single casing. One valve section controls the primary circuit, while the other controls the secondary circuit. When the driver presses down on the brake pedal:

1. The mechanical force of the driver's foot pushes down on a spring-loaded plunger.
2. This plunger moves a primary piston, opening the valve section for the primary circuit and allowing compressed air to flow from the primary reservoir to the rear brake chambers.
3. Simultaneously, as the primary circuit side pressurizes, this air pressure acts on a balancing piston inside the valve, which moves to open the secondary valve section. This allows compressed air to flow from the secondary reservoir to the front brake chambers.
4. If there is a complete loss of air pressure in the primary circuit, the balancing piston will not move via air pressure. However, the valve is designed with a mechanical backup: the primary piston will travel further and make direct physical contact with the secondary piston, mechanically opening the secondary valve to apply the front brakes.

This design ensures that even if one circuit is completely depleted of air, the driver can still actuate the remaining circuit simply by pressing the brake pedal.

The Flow of Air through the Dual System

To understand how the dual air system functions during a service application, it is helpful to trace the path of compressed air from the atmosphere to the brake chambers:

1. Air Compression and Storage: The air compressor draws in atmospheric air, compresses it, and sends it through a discharge line to the supply reservoir (commonly called the wet tank).
2. Filtration and Moisture Removal: In modern systems, an air dryer sits between the compressor and the wet tank to extract water and oil vapor. The wet tank acts as the first storage point, collecting any remaining moisture or sludge.
3. Distribution through Check Valves: From the wet tank, air flows toward the two main service reservoirs: the primary reservoir and the secondary reservoir. To prevent air from back-flowing if the wet tank or compressor fails, one-way check valves are installed at the inlets of the primary and secondary reservoirs. This guarantees that air can only flow into the service reservoirs, not out of them back toward the wet tank.
4. Feeding the Dual Foot Valve: The primary reservoir is plumbed directly to the primary supply port of the dual foot valve, and the secondary reservoir is plumbed to the secondary supply port.
5. The Service Application: When the driver depresses the brake pedal, the dual foot valve opens both circuits.
   • For the Primary Circuit (Rear Brakes): Air flows from the primary reservoir through the foot valve to a relay valve located near the rear axles. The relay valve uses a small pilot signal from the foot valve to quickly direct high-volume air directly from the primary reservoir to the rear service brake chambers, minimizing brake lag.
   • For the Secondary Circuit (Front Brakes): Air flows from the secondary reservoir through the foot valve to a quick release valve located near the front axle. The quick release valve speeds up the exhaust process when the driver releases the pedal, allowing the front brakes to release quickly.
6. Brake Application: The compressed air enters the service brake chambers, pushing against a flexible diaphragm, which moves the pushrod, rotates the slack adjuster, turns the S-cam, and forces the brake linings against the brake drum to slow the vehicle.