[martin417]

If you are trying to vary the force / speed of cylinders, there is a trick involving multiple cylinders. Instead of one large cylinder, use several small cylinder in parallel, each connected to its own valve.

To vary speeds: Connect all cylinders to the valves in the normal fashion. High force kick, extend all cylinders. Lower force, extend some cylinders, keep some in retract, these retracting cylinders will "fight" the extending ones, reducing the force and speed. A cylinder has more force in extend than it does in retract, so as long as you have as many or more cylinders extending than retracting, it will extend. If you use four cylinders, that will give you three different extend speeds / forces. We did this in 2010 (we also used surgical tubing to help extend, so we could actually extend with all but one cylinder in retract)