Update regarding the hiss. I've ordered a refurb kit for the filler cap from ceandess and waiting for it to arrive. I have since solved the issue of the hiss. I dismantled the locking mechanism from the cap, as when I pressed on the lock barrel, the hiss got much louder. After refitting everything back together, and making sure the o ring that goes around the lock barrel was properly seated, the hiss stopped and has never returned. I'm assuming it was just a natural escaping of gas from the fuel as it slowly evaporates. It certainly wasn't a huge rush of gas escaping.
