You should be able to look inside the brass block and see the angled seat that the brake line tightens against. If you see any distortion at all then then you have found the problem. Also push the nut all the way against the flare and make sure that the threads on the nut are square with the actual flare on the end of the steel line. As mentioned above, tighter isn't always the deal. If you have distortion your going to have a leak and usually if you clean everything up good and get out a magnifying glass you should be able to see why its leaking.
