OK, had another look at this.If I correclty understand, air is pumped into the rear of the assembly where I see the tyre valve cap (black). this forces the spool to seal off the breech as bob said.Correct. How have you configured the aperature through the spool into the pressure chamber?I take it when you dump the air in the "pilot" chamber, the spool moves back and the aperature is sealed off against the rear of the assembly due to the big washer? The big washer is serves a couple purposes. It takes up some of the volume in the pilot area and serves as a cushion for the spool to stop it's rearward travel. The spool opens FAST, and has considerable momentum when it's stopped suddenly. Without the bumper, it's very possible for the spool to be damaged.At first glance it looks like the pressure in the pressure chamber would force the spool against the walls...not back.Is the drop in pressure when you dump the air enough for it to move rearwards?Yes.You said the O-ring is floating. Why not groove the spool so the O-ring doesn't move? Wouldn't it work itself back eventually?I like your design. Very few moving parts and the diameter of your pressure chamber is the same as the barrel bore - i.e. no reduction of airflow.
I'm thinking of a sprung loaded ball bearing that protrudes slightly into the bore to act as a stop. As the bearing fires it simply pushes the sprung loaded ball bearing back. All sealed so no pressure escape.