You are correct that the shorter line is the one sucking air. The skimmer basket on each is relatively clean (as clean as they get with the current muck situation). The issue I have with adding a valve on the skimmer lines is that they do not come out of the ground separately. I do not know at what point they join, and I am not sure how much digging around I want to do to find out. The odd thing is that, the prior pump had other issues, but not this one. I went from a 1hp super pump to a 1.5hp super pump. Is it possible that the extra power is exacerbating the issue? I will try adding water to the pool, but isn't it typical that the water level is supposed to be halfway up the skimmer? I am currently even a little above that, and, if I go further, will probably be close to the top of the skimmer. I'm fine doing that if it fixes the problem, but I am wondering if there is something else going on that is causing this.