Hi @Rangerman . Obviously, it could be a number of things, including a leak/crack in the underground plumbing. However, there are a number of things you can look at first. I recently was getting air in my returns. I ended up finding a union that was loose. Keep in mind it wasn't loose enough to leak AT ALL, but with a pressurized system and the pump running it would pull in air. I would check all unions, as well at any gasket seals on pump lids, canister filters or automoatic chlorinators, etc. Take them apart, clean all conatct areas with a good degreaser, apply fresh silicone lube and check your returns again. If any of the O-rings appear "flattened" or misshaped or "grooved" from age or not being serviced frequently, replace them.
Try, leaving main drain open but closing your skimmer line, and then letting out air until the water is spurting.
You could have both. A large O-ring under the top (screws) then a small O-ring on the handle shaft.
