Dual Air Brake System and Safeguards
What the exam tests
This domain tests why heavy vehicles use a dual air-brake system, how the two circuits are separated, and what safeguards warn the driver or apply the brakes when pressure falls.
How the split system works
A dual system has two separate circuits, commonly primary and secondary, each with its own reservoir and lines. The compressor supplies both, but check valves isolate them so a major leak in one circuit does not empty the other. If one circuit fails, the other may still provide braking, but the vehicle must be stopped and checked; it is not a normal driving condition.
Safeguards
- Low-pressure warning: a light and buzzer must activate before pressure drops below 60 psi.
- Spring brakes: designed to apply automatically at roughly 20-45 psi, depending on the vehicle.
- Safety valve: vents at about 150 psi to protect the tank.
- Governor: limits normal system pressure, usually cutting out near 125 psi.
- Inversion valve: helps control spring-brake operation in some dual systems.
Worked example
A rear-circuit line ruptures. The gauge falls, the warning sounds, and the tractor may still have front service brakes for a short time. As pressure continues down, the spring brakes apply. The correct response is to stop safely, not to keep driving because one circuit still works.
Traps that cost marks
A dual system does not mean two compressors, and both circuits do not normally share one unprotected reservoir. The warning does not wait until zero pressure. Spring brakes are not meant for routine stopping. Do not assume a failed circuit leaves full braking; it leaves reduced braking and an emergency condition.