If I could make a possible last ditch effort suggestion (like all hope lost try to rma it or fix it), it might be possible that a quick cure liquid gasket (maybe automotive?) Might be able to seal that pump. Like I said it's just a theory. So it could be a final resort if you have no other options.
I think mine was sucking some air until at least 70% full, somewhere around there.
I wonder if the leaking is more prone to happening on high restriction loops. Cause that piece right there is from the outlet of the pump, so if the loop has high restriction there would be back pressure at that point as the pump tries to push the water.
By the way, tightening or re-seating the o-ring doesnt help?