I have a brand new pool. Pool equipment is 2-3ft above the pool. When I turn off pump there’s a slight bubbling noise that appears to be coming from multi port valve at sand filter. If you put your ear to the pipes you can hear water slowly bubbling and sounds like it’s draining back to pool. It takes several hours but all the water drains back to the pool and pump loses its prime. I’ve checked all the usual stuff: pump lid and O ring, used soapy water on all suction joints and unions, pump drain plugs. Pool installer said he couldn’t find anything and to just run pump 24hrs. Any ideas? Bad O rings in brand new multi port valve?