I have a 20000 gallon in ground vinyl pool with 2 bottom vents. One is in the bottom of the deep end and the other is on the slope just past the shallow end. When I blow these out, I get bubbles coming out of one (the vent with least resistance) and not the other. Since there is no way to seal that to blow out the other vent, will it be sufficient? I close the valve as I'm blowing the line to create an air lock. I assume most of the little water left will be well below the frost line? Any help would be appreciated.