Takes a little practice with the vacuum brake bleeder to get the hang of how much to open the bleeder vs. how much fluid and air you get out. I like to put on the hose over the end of the bleeder and pull a hard vacuum. Then slowly crack open the bleeder until you start to pull some fluid and maintain the vacuum. I usually just pull out one container full of fluid the first time (~4 oz.) then close the bleeder. Top off the reservoir and pull another few ounces, this time slower and with the bleeder just barely open to minimize air leaking around the threads. You can usually tell the difference in leaking air (will come in via fine bubbles) vs. trapped air in the brake like (it will come in in big bubbles, sort of like turning a bottle upside down and having the liquid inside pour out and the air comes in in big glugs).

Sounds like you may be opening up the bleeder too far if you are getting too much air. I've never used sealant on the bleeder threads, but maybe give that a try.