[Jeffrafa]

there is a much easier way....

you can simply use a three-position valve, it works just like a dual solenoid (and looks almost identical), but when you aren't sending electricity to either of the solenoids then it springs to center. This engages "all ports blocked" which basically means no air can flow through the valve at all

we are using two of these on ours, the only thing is you can't pulse power unless you have it give power just long enough for the cylinder to extend.

the advantage is that if you loose air pressure in you're entire system, then that device will still stay wherever you had it assuming it was in the center-off position. the only problem with this is that it takes air to actuate the solenoid (since the solenoid is like a mini-cylinder) so until you regain pressure on the incoming line you will not be able to release the cylinder from the position it was in.