Question Air tank drains: Answers Allow rain water to enter the tanks. Are used to drain water and oil from the tanks. Should never be opened. Must remain open while driving. Explanation In an air brake system, the air storage tanks are equipped with drains to allow water and compressor oil to be removed. The water and oil can damage the brakes if left to accumulate in the system. Take a FREE Practice Test Now This questions appears in the following tests: Delaware Knowledge Test Class B , Pennsylvania Knowledge Test Class B , South Dakota Air Brakes ,