I wanted to share our success with a lo-tech method for blowing out our main drain. This is the first year my husband and I closed our 24k gal, IG vinyl pool (in coastal RI).

We were able to blow out the returns and skimmer by attaching our shop vac to the lines without any problem. However our shop vac wasn't able to clear our main drain (didn't see any air bubbles from the drain). So we tried attaching our leaf blower. No luck. I had read here, and confirmed with our pool store guy, that the line for the main drain doesn't have to be completely blown out, just clear of water above the frost line. But how do you know this? We would have felt better if we saw bubbles coming our of the main drain to know that the water level was definitely below the frost line!

Fast forward a couple of weeks and I came across this thread http://www.poolforum.com/pf2/showthread.php?t=5912 mentioning a stopper with a tire valve stem in it to blow out lines using an air compressor. I found one for sale on-line (called "expand-n-purge"). I showed it to my husband, and he was able to make one by using one of our unused winterizing plugs (a 1 1/2" rubber stopper with a bolt/wingnut on it), and sacrificing a bike innertube for the valve stem. He cut out the tire stem and glued it into the hole where the bolt went through the rubber stopper (he used permatex, a silicon adhesive) and let it dry over night.

Today we blew out our main drain using this stopper and our manual bike pump! It worked great! It's a 2-person job (3 would be better actually). One person held the plug snugly in the pipe, and the other person (me) pumped the pump (you know the kind, a cylinder with a t-shaped handle, and you stand on the bottom of the pump stand to steady it). I was getting tired, and I asked my husband if he saw any bubbles, and he peeked through our fence and said "oh my gosh it's bubbling like crazy!" This is where the third person would be helpful, at the edge of the pool looking for bubbles. We closed the valve, took out the stopper and gave each other a high five.

Thanks again for the great ideas you guys generate, what a great resource!
Chris