Double ended cylinders? Self centering controls?

[pugs]

Another possible option, would require more work but is an option, Is to build a hyd rack and pinion.
http://www.hydraulics-pneumatics-eng...-actuator.html
The 3rd and 4th pics are what I'm talking about. This could perhaps be built and then hook a servo up to the input shaft and use the cylinder sections of it as the pump for the steering cylinder. Once air was bleed from the lines it should zero return right with the servo. This would be completly seperate system from the hydraulic pump and thus would work with out a pump running.

Just a thought anyway.