I have read the many posts about pool closing and have not seen this particular issue addressed - unless I've missed something. I have a skimmer box with two ports. One goes to the pump, the other goes to the bottom drain but is not connected in any way to the pump system. When I blew air into it with my shop vac a fine mist of silt floated out but no bubbles came up. When I used a shop vac to suction the water out it seemed that the water was coming up from the bottom drain as the line never emptied. - Could there be a flap in the line to prevent backflow? I've read elsewhere on here that a shop vac is not powerful enough to blow a bottom drain. If that is true should I use a compressor instead? Thanks for any advice you can give. I have got to get this pool done!