Why is it so crucial to drain your vehicle's air tanks at the end of each day of driving?

It is important that you don't let water and oil build up in the air tanks. If you do, the brakes may not work correctly. Each tank has a drain valve on it and you should drain each tank every day. If your tanks have automatic drains, they will keep most moisture out. But you should still open the drains to make sure. (According to CDL Handbook)