I have an 18'x48" Intex Ultra Frame pool with an Intex Krystal Clear 1600 GPH sand filter. I also use the Hayward Wanda the Whale automatic pool cleaner. This has worked great till recently we noticed the water in the strainer basket on the pump going low when the pump came on with the whale attached. The problem quickly got worse. Now it sucks enough air that the pump loses all pressure/prime after about a minute of running the whale. We have positively determined the air is being sucked in through the vacuum release valve on the plunger valve that's on the side wall of the pool. We had two more of these plungers (new) swapped them out, same problem. Three bad vacuum release valves? I'm guessing not! Any ideas what would be causing this valve to start sucking air when vacuum is attached. Pump works fine otherwise. We even took the Whale in for service, nothing wrong with it, no blockage. Backwashed filter (we're using Zeobrite ...) I took the strainer basket off the pump to see if there was anything trapped beyond there, nothing. Not sure where the impeller is on this pump. Can't seem to find a diagram anywhere showing the inner works so I'm a little hesitate to start taking it apart without some direction. Any ideas?