A sprinkler valve is a Diaphragm based valve with 3 "areas" the inlet port, the area above the diaphragm, and the area sealed off by the diaphragm...
When you fill your chamber it causes air to go into the inter port, but the air doesn't flow down the barrel because of a diaphragm that is held down by a spring... The air in the inlet port goes through an "equalization hole" and flows into the area above the diaphragm so the PSI in the area above the diaphragm and in the inlet port are the same...
And to fire the valve you rapidly drain the air that is above the diapragm so the diaphragm moves up because the pressure in the inlet port cant "equalize" fast enough, so it pushes the diaphragm and flows to the area that was sealed off by the diaphragm and out the barrel...
It's really 200x more simple than it sounds
Here's a sequence diagram of an UNMODDED sprinkler valve...
http://www.spudtech.com/content.asp?id=21
When yo mod a valve, you normally just put a larger exhaust valve that drains the area that is above the diaphragm... The most common valve being a blowgun.
