I have a 30 ft round AGP with hayward 250lb sand filter every time I turn pump off to backwash I get a GUSHER of water SPEWING out of my skimmer. How do I relieve this pressure or what is causing it? my 2nd year with this filter and last year had zero problems. Did I do something wrong when I shut it down last winter? Any help greatly appreciated. sorry forgot to add 1HP hayward super pump