Evap canister

I've got a toyota corolla 2001 with the 1.8L. It has the evap codes in the computer so we ran a smoke test through the system to see where the system is leaking and we found it leaking from the evap canister near the gas tank. We replaced the canister and cleared the light and a few days later the light comes back on with same codes. We decided to run another smoke test to see where the other leak is coming from and it ended up being the canister again. We replaced it with a new one and the same thing happened with the new canister. It took about a week for the light to come on but I'm wondering if theres someone out there that's run into the same proble.