Leaking slip fitting?

[Winton]

I've got a 2x4 Grimm, raised flue. The compartment that hangs on the side of the sap pan, where the float regulates the level of the sap and the sap comes in from the feed tank, is connected to the sap pan via a slip fitting -- just a copper stub that comes out of the compartment that slips into a copper sleeve in the side of the sap pan. Nothing threaded, or anything like that.

It leaks. A lot. Has anyone else had this experience, or does anyone have any suggestions for making it leak less, or maybe even not at all?

Thanks.