Fuel tank grommet

It may not be anything you have done, if the old seal was bad enough it may have been acting like a vent and the problem was there all along.
Go under the hood, find the purge canister, it will be on the passenger side under the air box on the frame rail, there should be a plastic hose running to it with a rubber hose connected to the canister, a purge valve with a hose to the intake. If this is all present then you could take a vacuum pump hooked to the plastic line, it should not hold a vacuum.
you may be able to rent a vacuum pump from autozone or buy one from harbor freight.