I think the vapor recovery system is the same for both the 6 and the 12 ?

you can remove the vapor buildup pressure problem on both tanks by looking under the front, passenger side fender (wing), for a plastic check valve connected between 2 hoses, each hose is of a different diameter. The check valve is mounted horizontally (left-to-right) on the inner fender wall, just before one of the hoses connects to the Charcoal Canister. (big container mounted behind the headlamp, in front of the tire). It would be best if you remove the right front tire.

Remove this valve and insert a 1/8" drill bit into it, pushing it and then pulling it through the opposite side. This will break the membrane and eliminate the vapor/pressure buildup once and for all. Reconnect the valve where it was. These GM valves never worked, a new one will do the same.

Or, if you don't want to damage the old check valve, just disconnect the hose between the valve and the charcoal cannister.

Jose

 

so the pressure buildup is now gone?? Glad to hear.

Jose