I have read and reread your section on auto drains. You refer to a auto drain with a float ball and drain.
Please excuse my lack of knowledge on this subject. I checked your references to vendors for the electric and float versions, but have not seen any reference to the automatic type that use the compressor cycle to drain it each time the compressor shuts off.
Harbor Freight offers a automatic drain valve that is plumbed to the feed line at the top of the compressor. It is spliced into the side of the line that is cutoff from pressure when the pump cycles off. It is usually a short drain cycle (3-4) seconds, but works each time the compressor reaches it's shut off setting.
I'm looking for your opinion and any experience with this type of auto drain valve. Pro's and Con's.
I have a older (20+ years) Craftsman 6 hp-33 gallon horizontal oil less compressor that was purchased new and sat for 5 years before I acquired it. I think I was the first user so about 15 years intermittent use. I plumbed it with this type drain valve about 10 years back. I always find a stain spot under the compressor where the drain valve is located, when I move the compressor to clean or sweep.
It seems to work OK. Occasionally I will hear it spit-discharge when the compressor starts up from being completely empty. The drain valve has never stuck open. The hoses and connectors will allow the compressor to drain the air completely out over a day or two. Other than having to completely fill the tank from empty each time the compressor has worked flawlessly.
Thanks for sharing your experiences!