EVAP Problem With Chevy Cavalier z24?

[Josh L]

I've had this cavalier for a while, and this evap problem always keeps coming back, error code P0440. I replaced the gas cap and it worked for a little while, but it seems my gas mileage always goes down after burning about a quarter tank. What else should I do or look for to fix the problem, because it just came back and I know my gas cap is tight, but what else can I check?