Leaking valve (kohler)
I had an issue like this once in a different application. What I did was add a washer under the nut, from the back side. You can usually get a thin rubber washer to stretch around the flare and inside the nut. Other options would be to make sure the nut isn't broken or been over tightened to many times causing scratching and scoring either on the nut or flare surface, or the flare being squished to many times and flattening it out or bending it strange. You could possibly even put a rubber washer between the flare surfaces of each pipe.