Air Brake System Malfunctions and Warning Devices
What warns you
- A low pressure warning light and buzzer must activate before pressure drops below 60 psi. On many vehicles one light serves both circuits.
- The dual air gauge shows each circuit; a needle that drops when you apply the brakes points to a leak in that circuit.
- Spring brakes apply automatically at roughly 20-45 psi, so a serious leak parks the truck for you.
- On some vehicles a wig-wag check is used: with the brakes applied, both gauge needles should move; one needle moving alone means a problem in the other circuit.
What to do
If the warning comes on while driving, stop as soon as it is safe, chock the wheels and do not continue. A truck that cannot build 85 to 100 psi in 45 seconds, that loses air beyond the leak limits, that has a pushrod stroke beyond the chamber limit, or whose warning device does not work is out of service.
Common faults
- Compressor or belt failure: pressure falls steadily, the warning sounds, the spring brakes apply.
- Governor failure: the safety valve releases continuously near 150 psi; stop and get it serviced.
- Frozen lines in winter: moisture in the tanks; drain daily and keep the alcohol evaporator filled.
- Dragging brakes or oil-soaked linings: heat, smell, or pulling to one side.
Worked example
During the pre-trip you fan the pressure down and the gauge passes 50 psi with no buzzer. The warning device is defective, so the vehicle is out of service no matter how well the brakes otherwise hold.
Traps
Do not treat the warning light as a suggestion, do not assume a silent system is a working one, and do not try to "fix" low pressure by pumping the pedal.