Pretty nice! But the piston appears to be stiff and hard to move... is it? Have you considered Ultra-high-molecular-weight polyethylene (UHMWPE) with floating O rings instead of the all thread/nuts/washers/rubber gasket setup? it would reduce the friction a lot and lower the weight.
I've been working on my firing mechanism for a few weeks now and it seems you need, quite literally, the reverse of what I need. It involves a charging handle separate of the hammer/spring and the sear.
This is how it works...
pull charging handle back,
charging handle pulls hammer/pin
sear catches the hammer/pin,
spring pulls charging handle back.
hammer moves forward,
hits the schrader - pilots the piston valve
If you literally turn it 180, it would pull the piston back for you hehe
I'm not sure what you meant by linking the piston with the breech loading mechanism. Can you clarify?