Main Drain Success Story

A shop vac won't have the pressure to blow a main drain. An easy way to tell how high of a column of water you can blow is to just stick the blow end of the hose into the pool and keep moving it deeper. When it stops bubbling that's the pressure in inches of water it can push. Chances are you will be lucky to get 2 feet down. You may have also noticed this but those shop vacs (the brand and the generic term) leak on the outlet side, probably intentionally, for motor cooling in case you totally block the outlet. I modified mine a little to make a better seal and get a little more pressure at the hose. As for what you do with a main drain? I know it's been discussed on this forum somewhere but I don't have one and never paid much attention to the posts.
Al