New Transmission Cooler line leak...

[slategrove]

Just bought new transmission cooler lines (and thought the stainless steel would last longer) so I spent the extra $ and got SS. Install was fast and easy (knew it was too good to be true) but now I have a small leak at the bottom line where it attaches to the radiator.

I know that stainless isn't going to be as soft as, say a brass flare, but I don't want to over tighten, but I unhooked it, wiped any debris that may have been there out, and re attached just to have a slightly slower leak still in the same place. It looks like it's coming from where the line goes through the fitting and also appears to be leaking from the seat that looks like it threads into the radiator.

Does someone sell the seat that threads into the radiator?

Is there something like "Stop Leak" that I can safely use in my transmission fluid?