Couple of things with these, Think its a 14mm Allen if I remember correctly, also try to use thread lock over ptfe as ptfe can damage the thread on brass and even sprise the fittings wide open if over tightened, also try to keep away from zinc coated male spigots (the black ones) as these can damage the brass in the female, use the stainless steel sutner or Kew ones.
There is o rings available for genuine sutner and Kew guns but not sure about the copy types. You need a special plunger tool for replacing them, I have one somewhere, bought it from Damien at q services, give him a ring and he'll fix you up with one
Hope this helps