So what this does, is that when you trigger a blowgun in the pilot side, the valve dumps more air through that blowgun (because there is no other way out of the pilot), which is just as effective as the blowgun itself.
The point of this valve is to be able to pilot a piston valve without turning off the air source.
But as far as I can see, there is still a path between IN and CHAMBER, even if you would install a check valve.
Actually, isnt this just a complex check valve? It allows air to flow from CHAMBER or IN to PILOT, and denies flow the other way around.
What you are looking for, my friend, is a slide-check valve.
Position one: Fill gun
Position two: Close up feed line and vent other side to atmosphere.
A slide check does that. They are relatively simple to build yourself (if you've got a lathe, its a piece of cake)
Added pic of slide check.