It could be that your collection canister is blocked somehow. Basically, there is a tube that runs from the tank to this little black canister that is located in the engine compartment on the passenger side near the smog pump. If this tube gets blocked, then pressure will build up in the tank. The tube to the canister serves as a "vent." Inside the canister is charcoal (I think) which reacts with the gas fumes somehow.
As you can see, I know little about how the whole process works, but hopefully someone more knowledgable can chime-in on this one.
I do, however, know that if you block the tube you will get pressure build up because I did when I blocked mine. I had to block the tube with a screw because I ditched all of the emissions stuff (smog pump, et al) on my coupe.
Hope this helps. Good luck.
E
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1991 5.0 LX Coupe - 38,000 miles
13.17 @ 106.14 mph w/ 2.138 60'
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