Are any air valves open that don't have something in the water. I have found in the past that having several open outlets would keep me from getting down to the filters. Is the 3/4 pipe new or have you used it before. When I set up my first run about 5 years ago I was told to use 1 1/4" and this is what I still use. Just wondering if the 3/4" restricts too much air? Certainly not an expert here though.