i have an above ground pool with a doughboy skimmer, 1.5hp proline pump and a sand filter with a mulitport valve. yesterday, i backwashed the filter until the sight glass was clear then rinsed for 45 seconds, then ran the pump for about 8 hours. the pressure didn't climb at all during this time, but when i turned the pump off water came back up out of the skimmer. so much water that the lid was pushed off the skimmer. does anyone know what would cause this to happen?