To answer your question yes there is a check valve between your pump and the water tank. it is right at the inlet to the pump. It's purpose is to keep water from backing up into your tank while hooked up to city water. Now as far as air getting into your system it is getting into it some place on the inlet side of your pump. Make sure all fittings are tight, do you have a antifreeze syphon valve attached to your pump? is so make sure the cap in on tight, I had mine leak air into the pump and yet it did not leak water when not in use.