I'm no expert but perhaps my experience will be helpful. My pump gradually took longer to prime after draining and filling. It got so I had to park on a slope and fill to overflowing on the high end. I told this story to the guy at the RV store and he said the rubber part in the pump needed to be replaced. I don't recall the name but it was a palm sized circular thing. Sure enough it worked perfectly with that new part.
I recently read a post here where someone had left his valve in the winterizing position for sucking antifreeze from a jug instead of from the water tank.
