Dual Air Brake Systems and Safety Features
This domain tests your understanding of the two separate air circuits in a dual system and the safety features that warn you or apply brakes when pressure is lost: the low-pressure warning, spring brakes, and parking brake control.
How the dual system works
A dual system has two separate reservoirs and circuits, commonly split front/rear or diagonally. If one circuit loses pressure, the other can still provide some braking, but stopping distance increases. Separate gauges show primary and secondary pressure. The system is not two complete brake sets at every wheel.
Safety features
- Low-pressure warning: a buzzer and light must activate before pressure drops below 60 psi. Treat it as an immediate stop signal.
- Spring brakes: held off by air pressure and applied by heavy springs when pressure falls to roughly 20–45 psi. They are parking brakes and emergency brakes, not normal service brakes.
- Parking brake control: usually a yellow diamond knob. Pull it out to apply; push it in to release. It controls the spring brakes.
- Tractor protection valve: protects the tractor air supply if the trailer separates or its lines fail; the trailer’s spring brakes then apply.
Worked example
While driving, pressure falls to 55 psi and the warning sounds. At about 40 psi the spring brakes begin to drag; near 25 psi the yellow knob pops out. The correct response is to stop safely before spring brake application, not to continue and let the brakes apply at road speed.
Traps
- Assuming a dual system gives full braking on every wheel after one circuit fails.
- Using the parking brake for normal stopping.
- Thinking the low-pressure warning waits until 0 psi.
Key points
- Dual systems have two separate circuits; one failure leaves partial braking.
- The low-pressure warning must activate before pressure drops below 60 psi.
- Spring brakes apply at roughly 20–45 psi.
- The yellow diamond knob controls the parking/spring brakes.
- The tractor protection valve protects the tractor supply and helps apply trailer spring brakes.