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CDL · Air Brake Parts

What is the function of an air dryer 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

To remove moisture and some oil from the compressed air before it reaches the air tanks, reducing water accumulation and freezing risk

  1. A To remove moisture and some oil from the compressed air before it reaches the air tanks, reducing water accumulation and freezing risk
  2. B To increase air pressure
  3. C To warm the air
  4. D To cool the engine

Why this is the answer

An AIR DRYER removes MOISTURE (water vapor) and some oil from the compressed air BEFORE it reaches the air storage tanks. CDL air brakes. This reduces water accumulation in the system, which: prevents corrosion and damage; reduces the risk of FREEZING in cold weather (frozen moisture can block valves and cause brake failure); reduces the need for frequent tank draining (though draining is still done). Some systems also use ALCOHOL EVAPORATORS to lower the freezing point of any moisture. Air dryers are common on modern trucks. The air dryer works with the wet tank and tank draining to keep moisture out of the brake system. Knowing the air dryer's function (removing moisture/oil to prevent water buildup and freezing) is commonly tested.
Source: FMCSA CDL Manual — Air Brakes, Air Dryer

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About this topic: Air Brake Parts

Knowing the parts of the air brake system — compressor, governor, supply tank, dryer, slack adjusters, spring brakes — is required so you can recognize defects during inspection. This is one of 22 Air Brakes questions on air brake parts, 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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