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CDL · Using Air Brakes

At what air pressure does the low air pressure warning signal typically activate?

Reviewed by Jordan Morley, Editorial Lead at 247SimpleTests. Written by the 247SimpleTests Editorial Team from the official source material cited on every question.

Correct answer

Below 60 psi (usually around 60 psi or below)

  1. A At 200 psi
  2. B At 125 psi
  3. C It never activates
  4. D Below 60 psi (usually around 60 psi or below)

Why this is the answer

The LOW AIR PRESSURE WARNING signal must activate before pressure drops below 60 psi (typically activating at around 60 psi or below) — alerting the driver that air pressure is getting dangerously low. CDL air brakes. The warning may be a light, buzzer, and/or a 'wig-wag' device. If pressure keeps dropping, the SPRING (emergency/parking) BRAKES will apply automatically (around 20-45 psi). The driver must STOP SAFELY as soon as the low-pressure warning comes on — never continue driving with low air. Knowing the warning activates at/below 60 psi is commonly tested.
Source: FMCSA CDL Manual — Air Brakes, Low Pressure Warning

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About this topic: Using Air Brakes

Air brakes feel different from hydraulic brakes — there is brake lag, and pumping defeats ABS. Hot brakes fade on long downgrades; engine braking saves them. This is one of 12 Air Brakes questions on using air brakes, each with a full explanation on a single study page.

New to this exam? Our CDL exam guide explains the format, scoring, and how to prepare.

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