no no, a check valve is not the 'thing' to control how much air flow. A check valve is to allow the gas to flow in one direction onli. this is to prevent presure from the reactorto 'push' the co2 gas back to the bottle. im using a DIY cos system too, i find the standard 1.5l bottle is a bit insufficient. im using the 5l bottlem, u know those mega big mineral water bottle? those are 5l. if ur using 1.5l bottle, i suggest u connect 2 bottles together to get the extra presure to 'push' the gas into the reactor.

bbtw, u can get a check valve in any lfs store, it's quite common now. go n check some of the lfs, im sure they have it. the last time i saw was onli 2 to 4 bucks. And SHAKE the bottle after u add in the yeast! and also shake the bottle gently everyday to make the yeast last longer, they will produce more co2 this way.

And make sure there is no leakage in ur system bcoz the preesure generated by the yeast is quite powerful, if there's is a leak in ur connecter or anywhere, the whole thing will be useless.