Failed smog test fuel tank not holding vacum

So my 76 Feather Duster runs great now. Took it to Smog test fail little to rich had new cat installed old one was empty yes like resonator empty. Crap gas cap test fail tank not holding vacuum . Crawled under car clamped vent hose still does not hold vacuum. So maybe sending unit seal or Filler neck donut. Any one does the vent tub in the tank have a gromet or is it welded in can tell its in the car. Dam Communist Californication Smog test:violent1::violent1::violent1::violent1::violent1::violent1::violent1::violent1: