) I've come up with a less sophisticated design as per the attached diagram.The firing cycle is as follows:
1) the firing chamber and piston unit start to fill with compressed air
2) when the force on the piston is greater than the force provided by the spring, the piston moves down slightly
3) this movement exposes the wider section of the piston to the force of the compressed air, pushing it down allowing the chamber to vent
4) as the pressure decreases, the spring pushes the piston back up and the cycle can continue.
The theory is sound but fleshing it out in practice looks a bit tricky as is, I'm still trying to refine the idea but just thinking aloud for now.
edit: A quick sketch of a possible refinement with the piston in two parts. The breech vent is sealed by a floating component, when the chamber is pressurised the piston component is forced down, eventually pulling the floating part down with it. The chamber is vented, allowing the piston to be returned by the spring and resealed. The advantage of this system would be that the main spring doesn't have to be strong enough to overcome the pressure, just enough to overcome the friction of the piston.
edit 2: here's the above idea as an animation
to remember 
