At what air pressure should the low air pressure warning activate, and what should the driver do immediately when it does?

Low air pressure warning activation: Per FMCSA regulations, the warning device (buzzer, light, or both) must activate before air pressure drops below 60 psi. Many systems activate at 60 psi exactly; others activate slightly higher.

Why 60 psi is the trigger: At 60 psi, the system is still above the spring brake application threshold (approximately 20-45 psi), giving the driver time to respond before automatic spring brake application. This is the WARNING window — the driver has time to act safely. THE GAP BETWEEN 60 PSI AND SPRING BRAKE APPLICATION: This gap exists by design — enough pressure remains for the driver to maintain steering and controlled deceleration while stopping. The gap is NOT for continued normal driving.

Immediate response: (1) Acknowledge the warning — do not assume it's a sensor error; (2) REDUCE SPEED — begin controlled deceleration; (3) SIGNAL AND MOVE to the right lane; (4) Pull to a safe stop on the shoulder or at a safe area; (5) Set the spring brakes (push in the yellow knob) BEFORE pressure drops enough for automatic application at speed; (6) Call for mechanical assistance.

Common causes of rapid air loss: Broken air line (often heard as hissing); brake chamber failure; compressor failure; fitting failure.

Consequences of not acting: If the driver continues driving and pressure drops to the spring brake range, the brakes will apply automatically and without warning at whatever speed the vehicle is traveling.