If you're using air brake line for air, then there are special fittings. They're not a lot different but just enough, and they're DOT certified. (I doubt anybody's checking these on an MV, it MAY be only for CMV applications which is pretty grey on these, but... they're easy to find and are no more out of line price wise than any other brass fitting) If the air brake line is used for anything but air, standard compression unions work just fine so long as you use the sleeve inside of the ferrule. That's the fuel line of choice on lots of new trucks. You can chaffe it, kink it, clamp it down under bell housings, and still the only way to make it leak is to actually cut it or to forget the sleeve at a fitting (which actually cuts it).