evap code P0440

I have a 96 chevy blazer LT 4x4 130000k. I keep getting code P0440 evap emision control. it will come on about a week after it is eased. Does any one know where I should start. I tried replacing the gas cap, but did not help. thanks

the same code keeps comming back. however, the last couple of times that I filled my gas tank I can smell gas inside the truck. the leak must be getting worse, but I can not seem to find where it is leaking. Since I can only smell it when it is full i would think that it would be the gas tank, but I don't want to just throw parts at it.