Advice: Understanding dual-circuit brakes

Today, thankfully, a dual-circuit braking system is a mandated requirement on all heavy-vehicle prime movers by various industry codes and standards, such as the European brake standard, UNECE R13, which coincidently meets the Australian Design Rule (ADR) 35/03.

The concept of the dual-circuit brake system is to eliminate a total brake failure if there is a fault with any part of vehicle's brake components. This will allow the driver to bring a heavy vehicle to a safe stop even if part of the braking system has a problem.

The main visual evidence a driver has that their vehicle is fitted with dual-circuit brakes is via the dash-mounted air pressure gauges, typically marked 'primary' and 'secondary'.

Related reading:
Advice: Understanding Trailer Response Management
Advice: Truck tyre pressures
Advice: Clutch brakes explained

However, as the name suggests, the dual-circuit brake system is made up of two separate circuits combined in one. On a typical 6x2, 6x4, 8x4 or 10x4 prime mover or rigid vehicle, one circuit operates from the primary reservoir and the other from the secondary reservoir.

If one circuit has a failure, the other circuit is isolated and will continue to operate, allowing the driver to stop the vehicle safely.

Most vehicle manufacturers typically have the primary reservoir operating the rear service brakes and the secondary reservoir operating the front service brakes.

The vehicle's compressor pumps air through an air-dryer if fitted to the supply reservoir (the blue tank), which has a safety valve to prevent over-pressurisation of the air-system. The pressured air is fed through a one-way check valve to the primary reservoir (the green tank) and secondary reservoir (the red tank).

From here the dual circuit starts, with air fed into the foot treadle valve from the primary reservoir, and also from the secondary reservoir by a separate feed line as can be seen in the treadle valve diagram.

The treadle valve is divided into two separate delivery ports, basically two valves combined in one. One port of this dual treadle valve controls air flow to the primary circuit and the other port controls air flow to the secondary circuit.

It is important to ensure the air pressure registers more than 690kPa in both service systems before you move the vehicle. If the pressure in either or both circuits is too low for normal brake operation, a warning light will typically illuminate in the warning light cluster and an audible alarm will sound.

By depressing the brake pedal it actuates the dual treadle valve, allowing air from the front and rear reservoirs to flow through their respective delivery ports. The primary air flows to the relay valve, which then draws air directly from the primary reservoir to the rear brake boosters.

At the same time, air from the secondary reservoir flows through to the front brakes. When the brake treadle valve is released, air pressure from all brake chambers is exhausted through the respective relay valves.

If air is lost in the primary circuit (rear brake), the check valves protect the secondary circuit (front brake). When the primary circuit pressure drops below 379kPa (55psi), the reservoir port in the spring brake valve no longer senses pressure.

When a service brake application is made, air in the front brake circuit goes simultaneously to the front brakes and the control port of the spring brake valve. The spring brake valve responds by exhausting the spring brake chambers through the relay valve, in proportion to the amount of pressure delivered by the brake valve.

Release of the brake treadle valve allows stand-off pressure to build up in the spring brake chambers and retract the springs; thus, modulated control of the spring brakes offers braking action similar to the service brakes.

If air pressure is lost in the front brake circuit, action of the check valves will protect the entire rear brake circuit from any loss of air. Only the front brakes will be inoperative. The rear service brakes will function normally. The rear brake circuit will continue to supply air to the trailer brakes.

The fitment of the dual-circuit braking system to heavy vehicles has greatly improved road safety because 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. Both the primary and secondary circuits are equipped with low-pressure warning devices and pressure gauges.

Conversely, in most heavy vehicles if the air pressure falls below 414kPa while driving, the spring brakes may activate and stop the vehicle abruptly, which could result in an accident and/or injury. For this reason, it is vitally important to monitor both air gauges.

On the other hand, if the red low air pressure warning lamp illuminates, indicating a sudden loss of air in the system, it is important to promptly find a safe place to pull the vehicle over safely and investigate the cause.

For more information on air brakes and the latest Australian Design Regulations, a copy of the Australian Air Brake Code of Practice that covers the latest information on ADRs can be obtained from the Australian Trucking Association.