What is the low air warning and approximately when does it occur?

Answer: B — An audible/visual warning that air pressure is dropping, typically around 55-60 psi.

In air brake systems, the low air warning is the signal that the system’s air pressure has fallen to a level where braking performance could be compromised. It appears as both an audible alert and a visual warning on the dash, so you notice it even if you’re not looking directly at gauges. The warning typically triggers around 55 psi, often described as 55–60 psi, giving you time to pull over safely and address leaks or other issues before brakes can be affected more severely. This warning is specific to the air pressure in the braking system, not to brake drum temperature, engine oil pressure, or tire pressure, which have their own indicators.