I am having vacuum problems. I turn off the bottom drain to start vacuum and after about 1 minute the filter at the pump empty's about half way and vacuum stops. I have 2 skimmers on is plugged. I put a piece of plastic in the other it comes right out at the. I have taken off the moter and cleaned the impellar and replaced the seal on the impellar. I have tried two different vacum hoses. The gasket on the filter looks ok but I have not replaced it. Any suggestions?