Did you try to loosen and than retighten the fitting a few times many times that will seat the flange on the fitting ,the tread on the fitting does not seal the line it's the fit between the line flare and the fitting that seals the brake line so it could be the flare or the fitting is bad or not lined up ,try to loosen than retighten frist ,you may have to reflare the line or replace the fitting look at both you should see where the two are fitted and see the problem.
Pete
