Emission Problem?

Hi,
I have a 1996, automatic, 3.8L V6 Pontiac Firebird with 104,000 miles. Here is the problem: after driving the car, when pulled into the garage, a very potent smell of gasoline fumes can be smelled (the garage is filled within an hour). I have done some minor things myself. The smell is originating from the back left wheel-well area. There is never any gasoline residue left on the ground and when looking from underneath the car, no gas residue can be seen on the underside of the gastank, etc. I took the charcoal canister off and it appears do be fine (it was really dirty however). I checked the canister by blowing in the fuel vapor in let hose and the vent is not stuck open. I've also replaced the solenoid valve on the engine and that did not affect anything. The pressure control valve that is in the rear of the car is working also. I received a P1441 error one time, but Autozone cleared it and it hasn't come back (but the car still leaks gas vapors somewhere). I took it to a mechanic and he hooked up a low pressure smoke system to it that is supposed to put smoke through the entire fuel system and if there is a leak, the smoke would come out of it. Of course, no smoke did. The mechanic said everything is probably okay. I still smell the fumes though and I know fumes are not healthy. I am thinking about replacing the canister since the smell seems to be originating from there but the part is $90, so if you have any advice, I would greatly appreciate it!