GM Repair Questions? Ask a Mechanic for Answers ASAP
You may have pinched the vent line from the top of the tank between the frame/ body of the truck. There is a vacuum line from the chracole canister that conects to the back of the fuel tank. IOf this line is pinched off then the evap system cannot pull a vacuumon the fuel tank.
this line will run to the front and rear of the fuel tank to the vents on each end of the tank. Make sure this line is not pinched off, or the line at the back of the tank that conects to the charcole cansiter. There is also a vacuum line from the canister that runs along side the fuel tank and runs to the enigne. Check this line as well to make sure it is not damaged.