Today I have been working on trying to get a better seal with the Robinson coupling. My original idea of putting a o-ring between the bottle and the nut is a utter failure. The o-rings are too soft and either get squashed out of the way or pushed through the hole. The rocket isn’t even water tight let alone air tight.
However I have been working on a new method today which seems to be working quite well. I am using bicycle inner tube to act as a rubber washer between the nut and the bottle. It seems to hold air up to about 30 psi, so its an improvement at least. I tried tightning the nuts some more, but they just seem to chew through the rubber. More research needs to go into this area but progress is being made.