My pump is a swimpro sw256t sand filter. When I try to backwash, the pressure is really low. When I rinse, the sight bulb has an air bubble in it. And there is very little suction at the skimmer. Yesterday I put a valve in the line that goes from the skimmer to the bottom basket. It was working ok right after that. I was able to vacuum. Now, there isn't enough pull at the skimmer to vacuum. Any ideas?