content: I guess it kind of makes sense because the depth of the water doesn't really increase rust.  If you drain it and the fill it right away there is going to be water. This is why I'm confused that no one does partial drains.  Keep it under pressure because that air is already dried instead of putting new air in.  Drain it till air comes out more frequently.  You'll be putting less air in so you are putting less water in, and getting rid of the water right away.