I uh... fill mine up my '65 c10 all the time, I've never had this issue since I bought it in 2003.
I also went to the hardware store and bought a BIG rubber washer from the plumbing section to fit onto my gas cap, as the paper/cardboard whatever that is was worn out. That might have something to do with it.
-W
update: after thinking on it some more... maybe I did have this issue when I first got my truck... but not since I put the big rubber O-ring on the gas cap there. The cap fits more snugly as well.
hardware store/plumbing department... take the cap with you.
Last edited by Woogeroo; 09/06/2009 8:03 PM. Reason: update
