Apologies, I'm getting my can issues muddled up in my head.
Yes, as metalmulisha stated above, the problem is having a blocked pin hole in the firing can, not too large a pin hole - that can cause efficiency and velocity issues, not cause your marker to fire when degassed.
Basically, as the marker is degassed, air from inside the front section of the can - which would normally hold the bolt back - is vented through the solenoid. Air in the firing chamber cannot vent through the solenoid, as the back check valve in the solenoid prevents that.
What should happen is that the PCV should slow air venting from the marker; that in turn allows the air in the firing chamber to slowly pass through the can's pin hole and replace the lost air pressure from the front of the can. The air pressure in front of and behind the bolt is constantly balancing, and so the sir in the firing chamber can't push the bolt forwards to fire the marker.
The problem comes if the pin hole is too large, or if it blocked. If the hole is too large, then when you are actually firing the marker, you'll lose more air through that hole as the marker cycles then you should do, and that will reduce efficiency, and possibly lead to low velocity. If the hole is blocked, when you degas the marker, air cannot pass from the firing chamber to the front of the can, and so the pressure does not remain equal as the air in front of the can vents. So when the air in front of the bolt vents, the pressure will drop, and the pressure behind the bolt will be able to push it forwards, and the marker will fire.