The filler neck grommets will leak when they get old. They harden like steel when they are old. If the grommet is in good condition it shouldn't leak even if you fill gas up into the filler neck.
There are some better repops out now that have a second lip on them. Seem to be a much better design.
I just got one of these and it is the good one.
Mopar 71 72 73 74 GTX Challenger Cuda Road Runner Gas Fuel Tank Filler Grommet | eBay
