Are you using this piston inside of a tee fitting? If that's the case, have a look at my piston valve from my hybrid.

For a pneumatic, you don't necessarily need the long rod and seal at the left end of my valve, although I imagine it would help the pilot dump quicker. You would just place your fill valve and dump valve behind the piston.
In your diagram, you appear to have an o ring around the front of the piston, near the sealing face. For it to open properly, air needs to be able to push by the front of the piston and exit the barrel.