Warns the driver that system air pressure is critically low.
In modern air brake systems, the warning is both visual and audible.
The warning activates when system pressure drops to 60psi (414kPa) or greater. In a number of provinces, the low-air warning must activate above 55psi (379kPa).
Why the Low Air Warning Activates at 60psi (414kPa)
The spring parking brake generate upward of 2000 pounds of pressure when compressed. To compress these springs in a Type 30 brake chamber, which has a diaphragm of 30 square inches, 60psi (414kPa) is required.
Do the math: If you multiply 60psi X 30in2 = 1800 pounds of force.
Therefore, when system pressure drops low enough to activate the low-air warning (60psi 414kPa), there is barely enough air pressure in the system to keep the spring parking brakes off.
Importantly, if the system air pressure drops low enough to activate the warning, the driver must get the vehicle off the road immediately and investigate the source of the problem. Do not continue until the problem is corrected.
Below 45psi (310kPa), the emergency spring brakes will apply automatically.