https://youtu.be/BZcMPOJa3ho?t=3m38s

This was the video I was referring to. At the 3:38 mark they show you the Z counterbalance cylinder. It does now look like there is a port on the bottom as well. At least there is an air line disappearing around the back in that general location. So its definitely a double acting cylinder. If its a double acting then it probably uses a special regulator.

There is a Thompson Pneumatic center here in town. I'll check with them tomorrow and see if they know what type of regulator would work for an application like this.

Did anyone else notice the Z axis drive motor? It looks like a Hybrid stepper motor. They made a point of using mitsubishi servo's on the other 2 axes. This is probably the reason why they need a balancing cylinder.