The shop manual isn't much help on this one.

Page 6C-2 shows a diagram that indicates that the fill tube contains 2 valves:

#3 is a "Roll over valve"

#15 is an "Evaporation shut off valve"

I'm guessing that #3 is the culpret that is causing your problems but the manual doesn't give any kind of troubleshooting steps. Nor does it provide any instructions to replace the valve. I'm guessing that the fill tube itself will have to be replaced since the roll over valve is probably an "unserviceable" piece in that part.

You may want to call Merlin to see if he stocks a "Roll Over Valve" for the fuel tank. That'll tell you if it's a serviceable item.

Keep us posted on what you find out.