I don't have any check valves, but I do have a valve on my sand filter. I just cleaned the pump filter and it went great. Maybe a quarter gallon of water was lost. This is what I did: I turned the sand filter valve to closed I then put a short piece of vacuum hose in the skimmer plugging the hole and let it hang over the skimmer outside of the pool. I opened the the pump filter lid and only lost the water that was in the vacuum hose. Sweet, no wet feet.