ORIGINAL: olnico
The problem with the hydraulic solution is the amount of elongation the system will take before the piston actually starts moving. This is mostly due to the elastic nature of the plastic tubing used. When the hydraulic fluid transmits the pressure from the command piston, the lines expand a bit and that creates some sort of hysteresis in the piston motion. Very difficult to control, especially considering the fact that the tubing elastic point is varying with the outside temperature...
Bingo - another problem that this system has. I think that is one reason that you need larger cylinders on the drive (servo) side than on the actuator side - you need more volume change on the drive side to get the same pressure on the actuator side in spite of the expansion of the plastic tubing...
I also think that no matter how you do it, without a bleed fitting, you're never going to get 100% of the air out of the system, so you also need the larger drive cylinders to compensate for the compression of the (small) amount of air that remains in the system...
Bob