Maybe a little help for you guys needing to stop the pressure build up without spilling fuel all over your truck. The pictures in this thread are being hosted and I strongly recommend you save them if you need them in the future because if the hosting place goes away the pictures will be lost. I don't save them because I don't have the space on my computer for all the pictures I've taken. This is a picture of the gas cap which should be very similar to what your looking at.
The two metal tabs hold the valve assembly in place. The two tabs get bent up to allow the valve assembly to come out.
The valve assembly removed from the cap looks like this.
This is what the bottom of the valve assembly looks like.
The rubber gasket gets removed to allow removal of the plastic ring which allows taking the assembly apart.
The plastic ring gets removed and the assembly looks like this.
Now down to the modification part. The large spring is the one which controls venting the tank and it's tension controls the pressure that the tank can build before it opens to relieve. The spring holds down this metal valve that has a gasket on the end. One end of the spring rides in the metal valve part and the other end pushes against the plastic hold down ring.
You'll also notice a white platic valve looking thing in the center. This is the vacuum breaker to allow air to enter the tank. No need to mess with this valve.
This is a picture of the original spring.
This is a picture of the modified spring.
Then you put it all back together and you can test it by blowing into the plastic piece and see if you like the pressure it relieves at. Just remember the spring must have a little tension to prevent gas from spilling on your paint. Good luck and I hope this helps with the gas tank pressurization thing.
Don Smith