Question Air tank drains should: Answers Only be used by mechanics. Only be used when it is wet outside. Be used to drain the tanks completely. Explanation In an air brake system, air tank drains are used to remove water and compressor oil from the air tanks. When operating a drain, be sure to drain the tank completely. Take a FREE Practice Test Now This questions appears in the following tests: Arizona Knowledge Test Class A , Oregon Knowledge Test Class A , Idaho Knowledge Test Class B , Louisiana Knowledge Test Class B , Iowa Air Brakes , Maryland Air Brakes , Virginia Air Brakes , Wyoming Air Brakes ,