I just closed my pool for the first time and have the skimmers and returns blown out and sealed. When I use my shop vac to blow out the main drain, as soon as I start the vacuum, I get bubbles out of the main drain for about 5 seconds. Then, no more bubbles come out no matter how long I leave the Vac on. I would think that it should continue to bubble if there is enough air pressure. I have just had it bubble for the 5 seconds and then closed the main drain valve. Is that good enough or do I need to see if a more powerful compressor is needed?