That's wierd :? Use the light to verify no bubbles in the others too??

You said you used flex PVC right? I'm not familiar with this stuff, but could it be contributing? Maybe if it is bent in too tight of a bending radius, cound it have a small crack that's giving you the venturi? Or maybe air trapped in the return plumbing - I have no idea how you would determine this, or fix it? What if you shut off the pump and let the siphon try to clear the return lines, but start it back up before the siphon breaks and give you more air in the return plumbing.

It is kinda fun sitting here trying to solve someone elses problem when I really don't have a clue :lol: