I've used these for years, and I leave them full, or part-full in the trailer for months. Never had a leak.... didn't even know leaks were possibly a thing. They are great. You do need the adapter, but it's a bit shit to use... I've found you can't just depress the filler the whole way, or fuel will squirt out around the adapter. A partial press seems to work. But I've hardly ever used an adapter - the Aprilia tank has a very big hole, so it's not required. And on my R1 I dremelled out the metal baffle inside the tank filler to enable full-flow quick fills