The pvc is stubbed through the concrete wall. The return port flanges are slipped over the pvc with a typical glue connection. It would seem that the failure is at this glue connection, as under air pressure when winterizing, with the outlet plugged, air leaks between the flange and the wall.

The return flange port, once installed, does have female threads, but that is just to accept an eyeball, or for plugging in the winter. Those threads, in this case, don't pertain to how it's installed.

So given this kind of setup, any idea as to how one would repair a leak between the pvc and the return flange - internally? Still use the hydrallic cement?