Question Air tank drains: Answers Allow rain water to enter the tank. Are used to drain water and compressor oil from the air tank. Should never be opened. Must remain open during transit. Explanation In an air brake system, the air storage tanks must be drained to remove accumulated water and compressor oil. Failing to do so can cause damage. Manually operated drains should be used at the end of each day of driving. Take a FREE Practice Test Now This questions appears in the following tests: Kansas Knowledge Test Class A , District Of Columbia Knowledge Test Class B , Delaware Air Brakes , Illinois Air Brakes , New York Air Brakes ,