Why do you need to drain air tanks regularly?

Question

Compressed air in an air brake system usually contains a certain amount of water and compressor oil. The water and oil can damage the brakes if left to accumulate in the system. Tanks must be drained regularly to remove this build-up.

Accepted Answer

Because compressed air contains some water which must be drained from the tanks

Answer

Because the air gets old and needs to be replaced

Answer

To allow for a change in air pressure

Answer

To remove bad air from the tanks