Dual Air Brake System and Safety Features
Why two circuits
A dual air brake system splits service brakes into two separate circuits, usually primary for the rear brakes and secondary for the front brakes. Each circuit has its own reservoir and check valves. If one circuit loses pressure, the other can still apply some brakes. A pressure differential warning alerts the driver.
Safety features
- Check valves and isolation: prevent a failed circuit from draining the good one.
- Low-pressure warning: must activate before pressure drops below 60 psi.
- Spring brakes: apply automatically at low pressure, typically between 20 and 45 psi; the parking-brake knob may pop out.
- Tractor protection valve: on combination vehicles, protects the tractor air supply if the trailer breaks away or a trailer line ruptures.
- Breakaway protection: trailer spring brakes apply if air lines separate.
- System gauges: show primary and secondary pressure; a normal reading on one gauge does not mean the other circuit is healthy.
Worked example
A combination vehicle's primary gauge falls to zero while the secondary holds 120 psi. The driver still has secondary service brakes, and the rear spring brakes may apply if their release air is lost. The vehicle is not fully disabled, but braking is reduced and trailer control may be limited. The correct action is to stop in a safe place and diagnose; continuing is unsafe. The check valve should keep the failed circuit from draining the other.
Traps
Do not assume one normal gauge means the system passes. Do not treat the parking-brake knob as a service brake. Do not claim the tractor protection valve repairs a leak; it isolates the tractor supply. Remember that each circuit provides only part of the braking, not a complete backup system.