Smog help!!!

[240trooper]

So I have failed smog twice due to the fuel evaporative control functional test failing. I've read a couple of threads and found that the test consists of testing the pressure between the gas tank and purge line to the charcoal cannister. I don't know much about the evap system but I wanted to know if the o ring that sits on top of the gas tank between the tank and the top that holds the fuel pump could be the culprit? A few months back my 255 lb pump went out and I replaced it with a brand new stock pump, I'm thinking maybe the o ring didn't sit right and that could be the problem? By the way I drive a stock 91 hatch ka24de. Im going to check it tomorrow but if that isn't the problem does anyone know what it could be? Thank you.