Not a double acting cylinder. A single acting , spring return, air cylinder with 2 ports. Most spring return cylinders have only one port but Norgren and SMC make a model with 2 ports. There are probably others, I just did a cursory Google search. Spring return cylinders have a spring inside them, they extend when air pressure is applied to the rear port. They retract when the air is exhausted. But if you connected the two ports together, the pressure on each side of the piston would be equal and cancel each other out and your Net force then would only be the force of the spring. As the piston traveled through its stroke, air would flow from one port to the other. If you put a flow controller there, you could control how fast it reacts by restricting the flow from one port to the other. I think the effect might be that small Z movements happen quickly but longer Z travel would take longer to equalize pressure. It might take a pretty large spring to counterbalance your Z axis. Maybe more than one. The spring return cylinders I've used have been on the small side. I've never tried this, only theorizing here.