What would cause my truck to trip the overflow protection device on the pump?

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Everytime I try to refuel my truck the overflow protection device on the fuel pump is activated for some reason. My problem started approximately a year ago with a check engine light with a code concerning my gas cap. I replaced the cap twice but this did not fix the problem. Now the problem is worse. I spend 20 minutes at the pump to pump 5 gallon of fuel. As you know, this beast drinks the fuel so I spend many 20 minute periods at the pump to get 50 miles down the road. How do I fix this issue? Dealer hasn't a clue, actually niether of the two dealers I have taken the truck to can help me. They want to do exploratory surgury on my dime. Let me know if you are familiar with this issue. Thanks, Bob