Not nerdy at all. I too had noticed that it hissed if I opened it the morning after opening it the night before, with no driving in between. I think I may need to see how it goes with the new cap - it could be that the pressure wasn't high enough for the valve to kick in.

There is a note in the owners manual referring to this:

"Open the fuel filler cap slowly and allow the pressure to release before fully removing the cap to avoid fuel spillage due to rapid expansion." 
Hung like Einstein, smart as a horse.

So, em, moving along then

moved to new topic

There is a note in the owners manual referring to this:

"Open the fuel filler cap slowly and allow the pressure to release before fully removing the cap to avoid fuel spillage due to rapid expansion."

'Allow pressure to release, but the original post was concerning a vacuum in the tank 11 GS Bali Blue
07 GS FL2. Usually muddy. Gone
Numerous LRs since 1963