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CDL · Driving with Air Brakes

What is 'brake lag' in an air brake system?

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

The short delay (about half a second) between pressing the brake pedal and the brakes actually applying, because air takes time to flow through the lines

  1. A The short delay (about half a second) between pressing the brake pedal and the brakes actually applying, because air takes time to flow through the lines
  2. B The brakes applying too fast
  3. C A type of brake fade
  4. D The brakes never working

Why this is the answer

BRAKE LAG is the short DELAY (about 1/2 second) between when the driver presses the brake pedal and when the brakes actually APPLY — because it takes time for the compressed AIR to travel through the brake lines to the brake chambers at each wheel. CDL air brakes. This is unique to air brakes (hydraulic brakes apply almost instantly). TOTAL STOPPING DISTANCE for air brakes includes: perception distance + reaction distance + BRAKE LAG distance + effective braking distance. The brake lag adds extra distance (at 55 mph, about 32 extra feet during the lag). Drivers must account for this added distance, especially at higher speeds. Knowing about brake lag (the air-flow delay adding to stopping distance) is commonly tested.
Source: FMCSA CDL Manual — Air Brakes, Brake Lag

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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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