I was fortunate to find the pin on my filler cap to have seized in the recessed (unlocked) position, and I only discovered this when I noticed the filler flap wafting about. Applied 3-in-1 oil spray which penetrated enough to get the pin to ultimately protrude, which I then attacked with emery to remove rust from pin, followed by thicker oil & grease. Since worked for last 3 years, and thanks, you've reminded me to oil it again - good preventive maintenance. Seemed a lot easier than trying to somehow remove device completely.
With the pin in locked position, there's just enough gap between the body and filler flap to see the mounting bracket (washer?) of the mechanism, and if your panel is stuck shut, then I'd have thought you might be lucky enough to free it by squirting in copious amounts of WD40 or Plus Gas in that vicinity. Certainly enough gap on mine to get the thin nylon applicator tube into the gap. Good luck
. They were both ok till we had all that snow. How does one get behind it to remove it so I can either repair ( glug it full of Plus Gas ) or replace.?? The pin does shoot back in occasionally, but not when I need the flap open to get fuel, 
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