SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/cdl-air-tanks-draining

Why must you drain the air tanks on an air brake vehicle?

To remove water and compressor oil.

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  1. To release extra pressure
  2. To remove water and oil ✔
  3. To test the governor

Why: Compressed air carries some water and compressor oil, which collect at the bottom of the tanks. The water can freeze in cold weather and cause brake failure.

Huh, didn't know that: Tanks with manual drains must be drained by the driver at the end of each day of driving.

The dad joke
Why did the trucker bring a towel to the pre-trip? He heard the air tanks had a drip.
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References

Answer: To remove water and oil. Compressed air carries some water and compressor oil, which collect at the bottom of the tanks. The water can freeze in cold weather and cause brake failure.

  1. California Commercial Driver Handbook (DMV) www.dmv.ca.gov/portal/file/california-commercial-driver-handbook-pdf
    Compressed air usually has some water and some compressor oil in it, which is bad for the air brake system.
    Checked 2026-10-10.

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