[ksafin]

I suggest you put regulator valves on both ends of the pneumatic piston itself and mechanically adjust the amount of air going in.

I don't know of a way to do it via code, but this is the way I think most teams would do it.

Looks like this: http://www.andymark.com/product-p/am-2032.htm
The dial lets you adjust the speed of air coming through the valve