I took the top off the sand filter and checked the o-ring. I also scooped a few handfuls of cottonwood cotton off the top of the new sand.
Retightened everything and backwashed, and vacuumed again. It was a tiny bit better, but the psi is still the same.........14 psi during regular skimming, and almost no bubbles.

During vacuuming it is still 9 psi, and the filter basket is still almost only half full. during vacuuming, the return seems to be blowing out a lot of bubbles, some big "gulpy" ones.

i'm still stumped. Where is the air coming from, and why is the pump basket only 1/2 full during vacuuming??

Frank D.