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CDL · General

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

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 brakes never working
  2. B 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
  3. C A type of brake fade
  4. D The brakes applying too fast

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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