Okay, I had the same issue with my (California) 2001 STS for almost 2 years.. After replacing every serviceable part of the evap system, (i.e purge solenoid, vent solenoid, canister, fuel pump seal, gas cap, and connection o-rings. It still triggered the p0440. So recently I built a smoke machine (from a youtube video) broke down each section of the evap system, and smoke tested it,and what I discovered was the filler neck itself was leaking at the cap location, not the cap, but between the cap & filler neck. There's an o-ring between the steel flange, and plastic filler neck that rots out.This is not a serviceable item ( however I did replace the o-ring on mine but that's tricky) you should replace the filler neck unit (with a new one) which is entirely accessible, and not difficult to do. Also check the clamps which attach the plastic filler neck to the tank.