I have a Doughboy as in my signature. When I connect my vacuum to the port under the skimmer I get air bubbles coming in through that port, even with the vacuum not attached. I do blow the line to the vacuum out from the return to get rid of all the air prior to connecting it. I get so many bubbles that the pump will run dry within a few minutes. Also, the suction collapses the incoming cheap black hose that comes from the skimmer. My skimmer lid goes down inside the skimmer to seal off the skimmer part and activate the vacuum port. The lid is transparent so I can see the bubbles coming in.
My thought is that the seal around the vacuum port is messed up since its only during vacuum that this happens. Any other ideas?