The captain then sees a fuel leak from a charted position and does not believe it is acceptable and wishes to reject the aircraft on that basis. He must either enter that leak in the documents as a new fault and await ground engineering examination to certify it as acceptable or not or, without doing any paperwork, he can (bravely) take his crew away and explain his actions to his Sqn Cdr. Although captains do have the final say, they are still part of a structured organisation and they have to answer for their decisions when they do not go with the plan.
AC, WADR that dilemma is common to all Captains, Military and Civil, Multi- and SE. What you characterise as "brave" I would contend is his/hers duty as captain. If you don't like what you see you don't take it, whatever anyone else says. It may lead to you wearing out innumerable carpets so what? You can walk away from them, possibly jobless, poorer, sadder, whatever. At least you walk away, and so can others! Of course I realise there are other considerations not mentioned here, of other lives at risk, operational imperatives etc. At the very least I would expect that would involve discussion not only with one's superiors, who won't be going, but with one's crew, who will be.