Dual Air Brake Systems and Safety Valves
Why two circuits
A dual (split) air brake system divides the brakes into two independent circuits so that a single failure does not leave the vehicle with no brakes. A common split is front axle on one circuit and rear axle(s) on the other; some vehicles split diagonally or by axle group. Either way, each circuit has its own reservoir and its own protection.
Key protective devices
- Check valves - at the compressor and at tank inlets, they keep air from flowing backward. A failed check valve can let one circuit drain the other.
- Double check valve - allows two sources to supply one device (for example, a trailer supply line or a parking brake release) without cross-feeding.
- Pressure protection valve - closes off a circuit that has lost pressure so the remaining circuit can still build and hold air.
- Safety valve - spring-loaded, on the supply tank, vents at a high set pressure to prevent tank rupture.
- Low pressure warning - buzzer and/or light that activates when pressure drops below a safe level (typically around 60 psi).
- Spring brakes - apply automatically when air pressure is lost, giving a fail-safe stopping force.
What a failure looks like
If one circuit fails, you should still have braking from the other circuit, but stopping distance increases and the low pressure warning may activate. Expect the vehicle to pull or brake unevenly. Questions often describe a gauge that will not build in one circuit and ask what the driver should conclude - the answer is generally that the system is not safe to drive until repaired.
Worked example
Suppose the primary circuit gauge holds steady while the secondary gauge slowly falls after shutdown. Because check valves and pressure protection valves should isolate the circuits, a steady leak in one circuit points to a failed check valve or a leaking component in that circuit, not normal behavior. The vehicle should be placed out of service until the leak is found.
Trap
Do not assume a dual system means "two separate brake pedals." It is one pedal acting on two isolated circuits.