The cap is a press on. With small ears. Two ways to hack it. Either get a small tube and pressure fill it with your already carbonated beer and put that yellow stopper back in it with the little t at the spout to the closed position. Which will fill and press the air our of the bag at the same time. Or, very gently pry the cap off after using and clean out the bag. Then, when you want to use, purge plastic bag with CO2 and expand it in the bottle. Fill with your precarbonated beer, press lid back on with the yellow spout in it an closed to seal it. The bags should wear our and it would really suck if the lids didn't stay attached. Be nice if someone made new press on caps with the seal for a dollar or two. It looks like it took a lot of force to push the cap on as every one has the "whitening" from pressure being applied. I've gotten 6 of them ready to try for a home brew. It is neat.