Dual Air Brake System and Safety Features
This domain tests the dual air brake system: two separate circuits, warning and protection devices, and the safety features that keep one failure from causing total brake loss.
Two circuits
Most modern systems split into primary and secondary circuits. Each has its own reservoir, check valve, and control path. If one circuit loses pressure, the other can still apply some brakes. The low-pressure warning light and buzzer must activate before pressure falls below 60 psi. The safety valve opens near 150 psi. The tractor protection valve closes automatically when tractor pressure drops to about 20–45 psi, trapping air on the tractor and preventing a trailer breakaway from draining the tractor system. The trailer supply valve (red) and emergency line control trailer air; the service line (blue) carries application pressure.
Spring brakes
Spring brakes are held off by air pressure. When pressure drops to about 20–45 psi, the springs apply the brakes automatically. They are the parking and emergency brake, not a substitute for normal service braking while pressure is adequate.
Worked example
A tractor’s primary gauge falls to zero while the secondary gauge remains near 120 psi. The warning should already have sounded before 60 psi. The driver should stop safely; the secondary circuit may still provide braking, and spring brakes may apply if their release pressure is lost. The system is not “doubled” in stopping power - it is designed for controlled stopping after a failure.
Traps that cost marks
Do not think a dual system doubles braking force. Do not put the low-pressure warning at 20 psi; it is before 60 psi. Do not say spring brakes are released by springs. Do not mix the red trailer supply line with the blue service line.