Early days on this, I've still a few tests to run. Noticing a drop in the spa water with equipment OFF, of about 1 inch every 3 hours. Not seeing any drop in pool level. Looking at some threads on the site, I'm seeing references to 'check valves'. On my set up on the pressure (post filter) side of the set up there is a Jandy that is usually set to allow water back into the pool and spa, it's set at about 2 o clock, so some water goes down the pool return and some goes down the spa return. Is the 'check valve' the spring loaded plastic assembly with a large grommet on it that is pushed against the spa pipe and has a screw thread holding it in position? Or is the 'check valve' the Jandy itself? Will closely measure pool level to see if spa water is being siphoned back into pool. Have not tried running with Jandy's set to spa recirculation only nor set to Pool recirculation only. I've only just confirmed there is something going on.
This is something like the beastie....it's under spring pressure and has a screw on cap at the end of the pipe.