I've used my Sears shop vac for years to blow the lines out with not problem. True, it does not blow air out the main drain, but I don't see any reason that you would need to do that. The ground (unless you live at the North Pole ) does not freeze down to 8 feet below grade. Typical frost line is 36-42" in most places, so as long as you push water down below that level you will have no problem. What I do after blowing lines is pour a gallon or so of mobilehome antifreeze down the line to further protect it. Then in the spring I just pump it to waste, although it will not hurt much if you get it in the pool by accident. I also pour a gallon or more down each skimmer line after blowing them out.