If memory serves, air in the system is from a leak in the intake side before the pump. That leaves two skimmers, main drain and all the connecting plumbing before the pump as suspect. I have unions on all three of those lines and I suspect those first. I lubed up the o-rings thoroughly but it did not solve the problem. Does anyone know of any tricks that they have used to locate source of leaks like this? It's not a lot of air, the system does not lose prime when it is off. Thanks in advance.