I had a similar problem with my first attempts at making a brewloon. I ended up cutting the balloon neck just after the valve where the channel narrows. The neck can be opened wide and the tube fitted without going near the valve flap.
That sounds a very good plan. I'll try that next time. I have some new, rather appropriate, completely naff and slightly more expensive balloons in the shape of filled beer mugs. Well it is the season to be jolly...........I had a similar problem with my first attempts at making a brewloon. I ended up cutting the balloon neck just after the valve where the channel narrows. The neck can be opened wide and the tube fitted without going near the valve flap.