That's why God gave us Stop Leak.

But.....

If it's the seam between the top tank and the core this may be a clue that your core is partially plugged. I had it happen once, it just kept opening up little by little. About 2/3 of the tubes were plugged (probably from Stop Leak!).

I guess I'm saying the epoxy might solve the symptom but not cure the disease. Solder would be my choice for a leaking seam.

R.