Like tractor air tanks, trailer air tanks should be kept free of dirt and oil by `_________`.

A Draining them every day.

explanation

It is important not to 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.