Yes, you could, but it wouldn't help with filling much. The current setup makes filling very easy with our air compressor--a single valve would just seem messy to work with. With the current setup, one valve does one specific thing; so one valve only lets air in, the check valve, and one only lets it out, the ball valve. No funky changing of fittings and what not is needed--it does everything it needs to do in one package.Also, after getting a closer look at the gun itself I have one more question. For filling, couldn't you just scrap the check valve, and instead use the ball valve? So rather than filling through one valve and venting through another, you could have the single ball valve doing both. This would save a lot of space and make the gun look more streamlined. Obviously you wouldn't be able to change it now, but it's just a thought.
Also, I'll try to add the linear output graph; I made the others for the article anyway, so it's no big deal.
@Soaker Leader: Welcome back!