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CDL · Driving with Air Brakes

On a vehicle with anti-lock braking systems (ABS), what should the driver do during hard braking?

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

Apply firm, steady pressure and let the ABS prevent the wheels from locking up — do not pump the brakes

  1. A Apply firm, steady pressure and let the ABS prevent the wheels from locking up — do not pump the brakes
  2. B Pump the brakes rapidly
  3. C Apply the parking brake
  4. D Take the foot off the brake

Why this is the answer

With ABS (Anti-lock Braking System), during hard/emergency braking the driver should apply FIRM, STEADY pressure and let the ABS do its job — the ABS automatically prevents the wheels from LOCKING UP (which would cause skidding and loss of steering control). CDL air brakes. DO NOT PUMP the brakes with ABS — pumping interferes with the system; just hold steady pressure. ABS helps you maintain STEERING CONTROL and a straight stopping path during hard braking, especially on slippery surfaces. ABS does NOT necessarily shorten stopping distance — its benefit is preventing skids and keeping the vehicle controllable. If the ABS warning light is on, the ABS may not be working (the regular brakes still function). Knowing to apply steady pressure (not pump) with ABS is commonly tested.
Source: FMCSA CDL Manual — Air Brakes, ABS Operation

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About this topic: Driving with Air Brakes

Driving a vehicle with air brakes is different from driving one with hydraulic brakes: there is brake lag of up to half a second, stopping distance includes that lag, the low-air warning must never be ignored, and spring brakes will apply on their own if pressure drops too far. These questions cover braking technique, stopping distance and what to do when the system warns you. This is one of 4 Air Brakes questions on driving with 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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