KNAVE

i'm planning on swapping my drums for teraflex discs. i was crawling under the jeep the other day just to take a look at what i'm up against, and i noticed that there was the beefy e brake line, a brake line with a hex shaped end going into the brakes, and a 3rd much thinner line going in that im not so sure about. my question is what is that third line, and if anyone has done a brake conversion where does that line attatch to the new brakes? thanks a lot for your help.