I read in a thread on CF that the plastic style drain valve on the water separator that comes on Airdogs does NOT drain water; and that Pureflow has recently switched to a metal drain valve in order to solve this problem. I was wondering if any of you had heard of this, and if it's a prudent move to get a new water separator with this new style drain valve on it?

What if there is no water to drain!

In 9 years of owning my truck with both stock and AirDog system (5 years) I've only seen water once (WIF light lit) in the all the years... I knew why because of the cheap mom & pop fuel station I got fuel at. But during my AD150 servicing I dump the entire contents of the WS filter in a bucket looking for water and never have. Also check the stock housing for water and never have found any again...

Yes it is very true. I am the guy who found and published the specs on the AD filters, and how little water they separated. Original AD water filters are rated at 20 GPH, 140um, 50% free water sep and 0% emulsified water sep. For reference the stock filters are rated at 95% free and 95% emulsified water sep. The current AD filter is the Luberfiner LFF9594 and is rated at 95% free water and 75% emulsified water sep at 20um at 90GPH. While its emulsified water sep falls below the OE level the AD f/w sep is pre-filter where 99.9% of water is free. So I would change your filter out for the Luberfiner at your earliest convince. Now on a side note, I am running 2 f/w seps on my truck. Both the filters are rated at 98% free and 95% emulsified water sep, and I haven't seen a drop of water in 30K miles. Modern fuel is much cleaner and drier than older fuel.. as long as the station does a good job once they receive it.