During the Air Warning Device Test, you continue fanning off air pressure by applying and releasing the foot brake without restarting the engine. By what air pressure must the low-air warning devices (buzzer, light, or flag/wig-wag) activate?

nhcdm-s.pdf Section 11M.1.2 - Air Brake Check, p. 11M-3; 49 CFR 383.111(b)(5)

The low-air warning is the driver's first alert that the system is approaching the point where automatic emergency braking will engage. Knowing the exact threshold lets a driver recognize a real emergency immediately rather than second-guessing a dashboard light.