If your CJ has the 15 gal tank, then it'll have an external liquid check valve. That's what prevents the gas from reaching the charcoal canister. IIRC, the 20 gal tanks had a built-in liquid check valve.
The top to the liquid check valve snaps on like a 5-gal bucket lid. If you're real careful, you can unbolt the valve, pop off the lid, and clean it out if there's something keeping the float stuck in the open position. It has a needle and seat system similar to a carburator. It should make sense once you have it out and opened up.
EDIT - I should have added... I just put one on my Scrambler after the exhaust pipe got bent up under the body when wheelin' some big rocks at PAP. The hot exhaust collecting in the corner of the tub melted/dissolved the LCV.
So I picked up another off a parts vehicle and opened it up and cleaned it out as a just-in-case.