Dual Air Brake System and Safety Features
The exam tests your understanding of the dual (split) system and the automatic protections built into it: the low pressure warning, the spring brakes, and the tractor protection valve. Expect questions on the order in which these act as pressure falls.
How the dual system is split
- A dual system uses two separate circuits with their own reservoirs, usually a primary (rear) and a secondary (front) circuit.
- Each circuit is fed through check valves, so a failure in one leaves the other with air and braking.
- A total loss of air in both circuits is what brings the spring brakes on automatically.
The safety chain as pressure drops
- Low pressure warning - a red light and buzzer must activate before the gauge falls below 60 psi.
- Tractor protection valve - on a combination vehicle it closes in the 20–45 psi range, shutting off the trailer supply line; the trailer's spring brakes then apply.
- Spring brakes - the parking/emergency springs apply mechanically when air pressure falls to roughly 20–45 psi, stopping the vehicle.
Spring brake control
- The yellow diamond-shaped knob controls the spring brakes: pull out to apply, push in to release, after pressure is up.
- Spring brakes are held off by air; they are the parking brake and the emergency brake in one.
- A modulating (dual) valve lets you feather the spring brakes for a controlled stop rather than a full lockup.
Worked example
During a test the gauge falls from 100 psi to 40 psi. Warning buzzer and light should already have come on (above 60 psi). At about 45 psi the tractor protection valve closes, and the trailer spring brakes set. If the tractor's own pressure keeps dropping into the 20–45 psi band, its spring brakes apply and the unit comes to a stop.
Traps that cost marks
Believing a dual system shares one reservoir; thinking the warning comes on at 20 psi; assuming spring brakes can be released without air pressure; forgetting that pulling the yellow knob is what applies them.