What does it mean when the low air pressure warning device activates while driving?

The low air pressure warning device — typically a buzzer/light combination — activates when system pressure drops to 60 psi or below. This is a critical safety emergency requiring IMMEDIATE action. WHY 60 PSI IS CRITICAL: the spring brakes (parking/emergency brakes) are held released by air pressure; when pressure drops below approximately 20-45 psi (depending on spring brake design), the springs AUTOMATICALLY apply the brakes; this will happen whether you want it to or not; if traveling at highway speed when this happens, the sudden brake application can cause a jackknife in an articulated vehicle; CORRECT RESPONSE to low air warning: (1) Check gauges immediately to confirm; (2) Safely reduce speed — don't panic brake; (3) Pull off the road as soon as safely possible; (4) Do not try to 'make it to the next stop' — spring brake application at speed is far more dangerous than a controlled stop; (5) Once stopped with spring brakes applied, call for assistance; (6) Do NOT attempt to release spring brakes manually without qualified help. CAUSE OF RAPID AIR LOSS: broken air line, brake chamber failure, compressor failure, or severe air leak. Check by listening for hissing and visually inspecting lines and chambers.