Dual Air Brake System and Safety Devices
Dual Air Brake System and Safety Devices
This domain tests the split system that lets a vehicle stop after one circuit fails. You need to know primary and secondary circuits, check valves, low-pressure warning, spring brakes, tractor protection, and anti-compounding.
Core rules
- A dual system has two separate reservoirs/circuits. The primary usually supplies the rear brakes; the secondary supplies the front. Check valves isolate a failed circuit so the other can still build and hold pressure.
- The low-pressure warning (light and buzzer) must work before pressure drops below 60 psi. Spring brakes begin to apply automatically around 20–45 psi.
- The tractor protection valve and trailer supply valve protect the tractor if the trailer breaks away or a line separates. The trailer emergency line charges the trailer reservoir and triggers trailer spring brakes on loss of supply.
- The parking brake valve controls spring brakes. Anti-compounding prevents service and spring brake force from being applied together on the same chamber.
- A dual system is redundancy, not extra stopping power. It does not shorten normal stopping distance.
Worked example
A driver loses the primary circuit on a straight truck. The gauge still shows pressure in the secondary, the low-pressure warning comes on at 60 psi, and the spring brakes apply near 30 psi. The correct response is to stop safely using the remaining service brakes and spring brakes, not to keep driving because one gauge still reads pressure. The secondary circuit may supply only front brakes, so stopping distance increases.
Traps
Do not think “dual” means two brake pedals or twice the braking force. Do not ignore a warning because the pedal still feels firm. Do not assume the trailer spring brakes release without tractor supply pressure.