In response to your PMed question (if it's not a question I specifically need to answer, it's better kept to the main forum):
Generally the pilot volume on a piston hybrid is vented by a "spool"-type piston with two sealing faces (one between the chamber and the barrel, and the other connecting the pilot volume to atmosphere).
This works pretty much backwards to a normal pneumatic piston valve - rather venting the pilot and triggering the valve by reducing the pressure behind the piston, rising pressure in front of the piston triggers the valve which then vents the pilot.
An example can be seen relatively clearly on MrCrowley's Mjollnir hybrid:
http://www.spudfiles.com/hybrid-cannon- ... 21873.html
Particularly pay attention to the two close-up pictures of the back assembly, with the threaded rod sections. This is connected to the main piston. When the chamber pressure rises after ignition and the piston is pushed back, the vent port is opened and empties the pilot volume.
The piston does therefore need to be manually reset after each shot (unlike with most pneumatics), as the pilot can't seal until the piston is in position, but it works.
(I think by those pictures point he'd moved to an O-ring sealing inside the vent, but his videos do show an alternative version, with a flat seal against the end of the vent).
Some people will add a pressure release valve to the pilot volume as a back-up (if, for example, combustible gas should get into the pilot volume and ignite), but that approach isn't normally used on its own.
If you know enough about the subject, that should give you an idea of the basic principles, but like I say, we don't have any guides on how to do this kind of thing.