i did this a while ago as i broke my breather tube off my clarke fuel cap so i bought a airline fitting sorry i cant remember the size but i just got the smallest that fits a breather tube
take the rubber gasket off, from the top drill out the hole working you way up to 12mm dont just go stright for a 12mm as its easier to keep the hole centered by working your way up, push the fitting in the hole so its stright then using a vice push the fitting in making sure its going in true and keep going till the bottom of the fitting reaches the bottom of the hole, remember to clean off any lose bits of plastic swarf and refit gasket cap, use a meduim size flat blade screw driver and gently push the edge back into pleace around the edge.
its not air tight around the hole but its really close to it so fuel will have a hard time getting out around the fitting the hole is more then enough for venting




take the rubber gasket off, from the top drill out the hole working you way up to 12mm dont just go stright for a 12mm as its easier to keep the hole centered by working your way up, push the fitting in the hole so its stright then using a vice push the fitting in making sure its going in true and keep going till the bottom of the fitting reaches the bottom of the hole, remember to clean off any lose bits of plastic swarf and refit gasket cap, use a meduim size flat blade screw driver and gently push the edge back into pleace around the edge.
its not air tight around the hole but its really close to it so fuel will have a hard time getting out around the fitting the hole is more then enough for venting



