[maxgebhardt]

I guess a mechanical solution would be the chinese finger trap idea, that maybe after the cylender extends, it extends passed a spring loaded switch. like how classroom doors are locked, so you cant turn the knob on the outside, but the latch can retract when you close the door. that would be the faisafe, but if you were off, you might bend the cylender, or it might get stuck. I'd stick with the double solenoid and leak prevention though.