Yeah...your barrel is the same size as the piston. Or in technical terms, your 'seat' is the same diameter as the piston's sealing face. Meaning there is no piston/seat area differential. Without this sealing face area exposed to pressure, the piston will have difficulties working.
I suggest you read more on WHY pistons work. What makes a piston valve work? Ask yourself that question, and you will see that you have no unbalanced force, forcing the piston back when the pilot volume has been exhausted.
Edit:
That's a diagram 'thedeathofall' made for another topic, it shows the flaw in your design that I talked of.