spud gun/T-shirt gun valve

[Mykey]

I will not repeat the value of having the correct materials for the job - safety first!

That being said, many of the valves we are talking about have a diaphragm in them that is held in place using a spring. If you change (lower) the pressure applied by the spring it results in a more abrupt and greater release of air flow. If you lower it too much then the valve lets go on its own when the pressure reaches the lowered break point.

So... replace the spring with a weaker one. Or the "poor teams" method is to clip small pieces off of the spring bit by bit until it has the pressure setting you want.