I found this post via the "search" mode...
I noticed tonight that my 1974 450SL has the exact same problem. My car will leak gas when I drive it or rev the engine, but does not leak gas when it is sitting there. I could not see where there is any place where the tank could be leaking, however when I took a good look at my gas cap it looks like the seal on the cap is kind of bad.....so off to the Benz dealer tomorrow....I'll probably have to order one....
1974 450SL 99K miles
1996 C220 142K miles
MBCA Concours Judge
Western Reserve Section