I have a main drain and skimmer. I've been closing my own pool for the past 10 years. What I do, is put my filter in waste position and attached my Mighty vac blower to the waste pipe which blows air in reverse direction into the pool. Within about 20 seconds the bottom drain begins to enrupt with a 3 foot gieser of bubble water. Then, I slowly turn the gate value to close off the air going into the line. Once closed the water can't back into the bottom line.

Mighty vac is here: http://www.poolsupplyworld.com/pools...FROe4AodJVQAiQ