Could make the spring mechanism more compact.

That's what I thought too, but I have no idea how I could do it. If I would shorten the spring, the valve would not open fast enough.

First point shouldn't be a problem because I will use a 1/2 inch valve. 2. I didn't took that in consideration.

Thank you for your comment.

This is my idea about a new trigger system. The red line is the handle and the green is a torsion-spring from a mousetrap. You pull the trigger to close the valve and then when you leave it the valve will open very quick, like a mousetrap.

This idea currently only exists in this post. So before I ...