Originally Posted by Baph

Speaking about my bike:

I personally fill right up to the lip, with the bike upright. I've never seen it leak when it's on the side stand, so depending on age, perhaps the seal around the edge has perished? My bike hasn't even leaked fuel when it's been laid on its side in the middle of the road.

Personally, I'd look for someone breaking the same bike as yours, and try tempt them to part with the filler cap as a complete unit. Then strip it down & replace your lock with the "newer" seals from the replacement. That way, you still only have one key to worry about. Otherwise you'll be buying a new lock, which will also come with a new key.

To my thinking, there's an over-spill pipe from under the cap, so the caps should be air/water tight, allowing air in from the breather tube.