I’ve got a great design for a check valve now. I’m back to using bicycle innertube, since the butyl makes an excellent airtight seal.
First, cut a section off the innertube. You’re going to use the inside of the innertube for the valve, and you need to pay attention to making it as smooth as possible.
Identify where on the inside is the least smooth: on my tube, there’s one seam in particular that makes an indentation on the inside, so I cut there. Cut the section lengthwise so it stops being a tube and starts being a flap.
Next, you need to wash the tube with soap and water, to get the talc (I assume) off the inside. I think they put the powder in there to keep the tube from sticking to itself when folded. So get that off there, then dry it, so you have a nice smooth surface.
Hold your tube (blowpipe or intake valve tube) against the flap, and use it to draw a circle on the butyl. Cut the flap to the circle you just drew: it will be just a tiny bit larger than the tube, which is fine.
Mark on the tube where the flap is going to be anchored: you’ll want to be able to identify this when you insert it, so gravity can help pull the flap shut. Apply a little bit of vinyl adhesive to the top of the tube, trying not to have adhesive against the inner bore (it can prevent a good seal). Then press the butyl flap against it, and you’re done! Put the piece flap down against a flat surface, and try to put some pressure on it as the glue cures.
Next time I make one of these, I’ll take photos, and update these instructions.