I think you really need to narrow down what the problem is. You could test the steering cylinder using just air pressure to see how much pressure was required to move it when the aircraft is standing still and when it is moving. Once you have that nailed down, then you could work on sizing the drive cylinders and servo to provide you with that amount of working pressure. With a large BVM or Robart cylinder and an 8711 servo, you could get a heck of a lot of pressure.

Also, you might find that the steering servos are not big enough to move the nose gear with a reasonable pressure (over 150 psi or so and you're going to have to worry about the tubing blowing off the fittings or splitting) and if so, you'll need to see if its possible to mount a bigger cylinder on the gear.

I would think that its a solvable problem with a step-by-step approach...

Bob