If there is a crack in the end tank, replace the radiator, do not waste your time playing with epoxies.You hope it is there. Try that fix, it may work. Use a double wrench system with a flare wrench. Do not overtighten. But often it is the plastic tank that has a crack at the fitting. Then you get to try JB Weld or all manner of fixit type products but usually requires a new radiator which may not be a bad idea if it is original or high miles. Good luck.