I'm guessing it was most likely your charcoal canister rather than the check valve gasket. If the gasket were bad you would smell gas every time you filled up. Side note, replacing the charcoal canister and not the gasket at the same time would most likely result in that gasket failing shortly after. So it's a good thing you replaced the gasket during this process.

Also, there are a couple different part numbers for the rear charcoal canister, and it's important that you match the correct one to your vehicle. If you use the incorrect one you'll have a hard time starting it the first time after filling up, and it will progressively get worse.

Mike