I use to drain mine but have not in a long time. You are supposed to to keep from the inside of the tank from rusting. We had a compressor on base where I use to work that ran the HVAC system that was a 15 gallon style compressor, I drained it one day just to see and got over 5 gallons of water out of it. It never was drained in all the years it was in use. 
I was looking at hooking up a drain system to mine. More for safety reasons as well as to keep from rusting the inside. Not to mention I'm to lazy to drain it manually.
I found this online and thought it was pretty interesting.
I was looking at hooking up a drain system to mine. More for safety reasons as well as to keep from rusting the inside. Not to mention I'm to lazy to drain it manually.

I found this online and thought it was pretty interesting.
