Dual Air Brake Systems
The Dual Air Brake Systems domain tests your understanding of split-circuit air brakes. A dual system has two separate circuits, usually called primary and secondary, so a failure in one circuit does not leave the vehicle with no service brakes.
How the circuits are arranged
- Each circuit has its own reservoir, check valve, and lines.
- The foot valve sends air to both circuits, but the circuits are isolated from each other.
- A common split is primary for the rear brakes and secondary for the front brakes, though some vehicles use a different split. Know the vehicle you drive.
- A gauge or gauges show pressure in each circuit. Some vehicles use one gauge with two needles.
- If one circuit loses pressure, a check valve or isolation valve prevents the other circuit from bleeding down through the failure.
- Spring brakes apply automatically when system pressure falls too low, usually in the 20–45 psi range.
Worked example: You notice the primary gauge falling toward zero while the secondary gauge holds near 90 psi. The service brakes still work on the secondary circuit, but stopping distance will be longer and the vehicle may pull. This is an emergency condition: stop safely, do not continue the route, and have the system repaired.
Traps: assuming a dual system gives normal braking after a failure; thinking both gauges must always read the same; forgetting that a rapid loss in one circuit can still trigger spring brakes if pressure continues down. Never “test” a dual system by deliberately draining a tank on the road.