When I close my pool I connect my compressor to the pump drain plug and push air into the system. I have 2 skimmers, 3 returns, and a main drain. I do not lower the water.
The skimmer blow out first, one then the other. I blow out the lines, put a gismo in each close the union ball valve after about 1 minute or so and then anti freeze in the pipe through the gismo and skimmer basket area. Once the skimmers are done the returns start blowing air (again 1 at a time). I use the threaded plugs. I use the teflon tape, a little gasket lube and screw them in. After the last of 3 are plugged the air starts coming out of the main drain.
The 3rd return (last to plug) still has a few tiny air bubles coming out at the same time. Let's say maybe 2mm in wide and one a second. Do I need to worry about that? I know if I lowered my water I would never know it wasn't 100% sealed. I get enough rain and snow that at some point the water would come up over the return anyway.
So here is my question. Once the compressor is turned off the air bubles stop (of course) but I am wondering if water from the pool will get in, fill the pipe up and cause problems.