It's a measurement thing. You need to make sure that there is a little tension on the spring to seat the valve when no pressure is there. You can check this by trying to blow through the valve before you do anything to it. I'm sure you'll find it hard if not impossible to unseat the valve. Most people are not designed to develop much pressure. There are some exceptions.
When you are modifying the spring remember that the distance between the plastic retaining ring and the metal valve is the distance that is the minimum the spring height must be. You really want to have some compression of the spring but not much. When you put it all back together again. Check the valve by blowing into the holes on the tank side. You should hear and feel the pressure relieve. If you suck on the valve the vacuum breaker will allow air to flow into your mouth. Just remember gas doesn't taste very good. You might want to wipe it down before doing this.
Don