BlenderTek shows how to rig pneumatic systems in Blender.
An aside from my series on modelling and rigging an entire crawler hydraulic excavator from scratch for a sandbox video game, I made a tutorial video for the latest Blender version, updating on Johnathon Williamsons' VERY old video.
In 15 minutes I show how to setup a full rig for any kind of cylinder/piston system like a hydrualic or pneumatic ram using strech-to bone constraints.
Hope it helps people out because I constantly see people asking how to rig mechanical models on BlenderArists and the like.
-The BlenderTek.com Team