1.2 Air Pressure Warnings and Spring Brakes

A low air pressure warning (light and/or buzzer) must come on before pressure in the tanks falls below 60 psi. If you see it, stop safely and park as soon as possible — you may be close to losing brakes.

Spring brakes are the mechanical backup: powerful springs held back by air pressure. If pressure falls too low (typically in the 20–45 psi range), the springs apply the brakes automatically. Spring brakes are also what hold the vehicle when you use the parking brake control.