Compressed air in an air brake system usually carries some water and compressor oil along with it. Why is it important to drain the air tanks regularly?

Explanation

Water and compressor oil collect in the bottom of an air tank, and in cold weather that water can freeze inside brake valves and lines, which can cause the brakes to stop working; draining the tanks regularly removes both contaminants before they cause a problem. This is one of the few air brake facts directly required by federal regulation rather than just topic-scoped: 49 CFR 383.111(b)(2) specifically requires CDL applicants to know the dangers of a contaminated air supply, dirt, moisture, and oil by name. Recharging the compressor's oil supply is wrong because tank drains remove air-tank contamination, they do not add oil to the compressor. Resetting the warning light is wrong because that light responds only to system air pressure, not to whether a tank has been drained. Requiring drains only on vehicles with a front brake limiting valve is wrong because tank drainage is routine air-system maintenance on every air-braked vehicle, unrelated to that separate, largely obsolete valve.

Source: 49 CFR 383.111(b)(2)