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CDL · Using Air Brakes

If the low air pressure warning comes on while driving, what should the driver do?

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

Stop safely as soon as possible and park the vehicle, because continued air loss will cause the spring brakes to apply (possibly suddenly) and the vehicle could become uncontrollable

  1. A Pump the brakes repeatedly
  2. B Ignore it
  3. C Stop safely as soon as possible and park the vehicle, because continued air loss will cause the spring brakes to apply (possibly suddenly) and the vehicle could become uncontrollable
  4. D Speed up to reach the destination

Why this is the answer

If the LOW AIR PRESSURE WARNING activates while driving, STOP SAFELY as soon as possible and park. CDL air brakes.

Why: continued air loss will eventually cause the SPRING (emergency) BRAKES to apply automatically (around 20-45 psi) — possibly SUDDENLY and unexpectedly, which could cause loss of control, especially at speed. The low-air warning means the system is losing air faster than the compressor can replace it — a serious problem. Do not continue driving hoping it resolves. Stop, park safely, and address the problem. Knowing to stop safely immediately when the low-air warning comes on is critical, commonly tested safety knowledge.

Source: FMCSA CDL Manual — Air Brakes, Low Air Response

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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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