Failed smog test fuel tank not holding vacum

vent tube stub is solid to tank then short rubber line to metal fuel/vent line., Sending unit possible, but would probably leak gas. I would think neck grommet likely if you think the cap is sealing. That should be same as my 75 - vent line is 2 piece with a rubber hose connecting near passenger floor, then up to charcoal canister. Be sure all are attached.