Then your air trap was deficient. The aluminium core needs to be set into a vice if you want to remove it from the bottle. If it is not the case, you need to wrap one or two turns of teflon bands into the oring groove.

Once again, the plastic cap is not meant to seal the bottle.
I have forgotten it once after a fuel maintenance job on the Mystere. The flight was unevetful and I only realized my mistake because the core popped out during the subsequent high pressure refueling.

There is no possibility of air leaking through the aluminium top. All the nipples are screwed into the aluminium for a thickness of 1/3" and sealed with a fuel resistant glue.