Here are the blueprints my dad and I used. This will give you guys an idea of the design and what we have to work with. I'm terrible at drawing so this will be a better diagram.
For the pressure valve, we added a T fitting between the air chamber and valve with a 90 street elbow. The pressure relief would sit on top between the barrel and air chamber. I'm thinking about doing the same setup on the upgrade with all new metal fittings.
We used a tire valve to fill the air chamber and use a cigarette lighter air compressor used for car tires, bikes, blow up toys etc. It doesn't work too well and overheats because it's not made to handle that much output. Anyway, if you look at the diagram on the website I posted above, you will notice that everything is going to be the same except for the bottom part (air chamber). You will also notice that when I use the fire extinguisher, the air chamber will be much shorter than the barrel, possibly making my firing mechanism a little awkward since it's so close together.
I use a Rap4 tactical 10 vest that has my tank on the back and I'll run my remote hose to the CP regulator and quick disconnect. The reg is a little awkward since it is about 4-6in long, when screwed into the ASA. So my dad suggested mounting it on the tank and in my pack to get rid of the extra weight.
BoomMaster, I've seen your backpack design and really liked how you did all your plumbing in the back, however, for my current setup I don't think that will work very well since my tank is on my back. I want mine to be shoulder fired like it was before.
I want to reuse the sprinkler valve because it works and I've attached a cool little pigtail as a safety feature to open and close the valve from my doorbell switches, that is already attached to the valve itself. So, I'm only wanting to replace the air chamber.