Dual Air Brake System and Safety Features
Scope
The Dual Air Brake System and Safety Features domain tests the two-circuit design that lets a vehicle stop after one circuit loses pressure. Questions cover primary and secondary circuits, spring brakes, low-pressure warnings, tractor protection, trailer air supply, and safe responses to a failure.
Core rules
- A dual system has two separate circuits, usually primary for the rear brakes and secondary for the front. Each has its own reservoir, check valve, and lines.
- If one circuit fails, the other can still provide braking, but stopping distance increases and a warning should appear. Stop and fix the problem; do not continue a trip.
- Spring brakes are mechanical parking and emergency brakes. Air pressure holds them off; when pressure falls to about 20–45 psi, the springs apply the brakes.
- The tractor protection valve protects against a trailer breakaway. It closes when the trailer air line separates or pressure drops, helping keep tractor air from being lost.
- The red trailer air supply valve controls air to the trailer. It is not a service brake and must not be used for parking.
- A low-pressure warning must stay on until the problem is corrected.
Worked example
A primary circuit line ruptures. The gauge falls past 60 psi and the warning sounds; the secondary circuit still applies the front brakes. As pressure reaches about 20–45 psi, the rear spring brakes apply. The driver can stop, but should expect a longer distance and should not try to drive to the next town.
Traps
A dual system does not double braking power and does not give two pedals. Spring brakes are not for normal stops. Do not fan the pedal to “build up” pressure; find the leak.