This maindrain purging stuff has got me thinking. Even tho I'm pretty sure I do an adequate job when I close by blowing out with line a shop vac and quickly slapping a rubber plug on and duct tape, I have to admit to occassionally, at the coldest times of the year, wondering if my pipes are silently succumbing to the ravages of frost. A 35' run from the pump to the pool only adds to my anxiety. I know those lines are not below the frost line.

What I'm thinking I might try this year is to secure my vac to the pump end of the MD line, run it till bubbles emerge, and move a piece of heavy gauge poly into place over the drain. Turn the vac off and allow the pool water pressure to press it into place covering the drain. With enough overlap, I ought to get enough seal.