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CDL · Air Brake Parts

What is the purpose of the supply (wet) tank 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 be the first reservoir that receives air from the compressor, allowing moisture and oil to collect before air goes to the service tanks

  1. A To be the first reservoir that receives air from the compressor, allowing moisture and oil to collect before air goes to the service tanks
  2. B To store fuel
  3. C To cool the brakes
  4. D To hold brake fluid

Why this is the answer

The SUPPLY TANK (also called the 'wet tank') is the FIRST reservoir that receives compressed air from the compressor. Because air from the compressor carries MOISTURE and OIL, the wet tank lets these contaminants COLLECT/settle out before the air passes on to the service (dry) reservoirs that operate the brakes. CDL air brakes. This is why it's called the 'wet' tank — water and oil accumulate there. The wet tank must be DRAINED regularly (it collects the most moisture). Many systems have an air dryer to remove moisture before the wet tank. Knowing the supply/wet tank's role (first reservoir, collects moisture/oil before service tanks) is commonly tested air brake system knowledge.
Source: FMCSA CDL Manual — Air Brakes, Supply (Wet) Tank

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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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