Yes, isolate the two systems. And also do what ame said. With the tires off the ground you should be able to turn your steering wheel easily even without any assist. I know you have big tires, but I forget what your linkage is setup like.
But the real reason I chimed in at all (since I have zero personal experience with a setup like yours) was to find out if your gearbox is just ported and capped, or if you have a steering ram connected?
If you have a ram, is it possible that the lines are backwards?
What made me think of this is that it got harder to steer as pressure went up. With ram-assist, as opposed to full hydraulic, your gearbox is mechanically linked and only going to work one way. Whereas the hydraulic ram could be pushing against it if plumbed wrong.
I'm not familiar enough with the assist setup to know if this is even possible. But it sounded plausible and the only thing I could think of for why it would get harder as pressure went up.
Paul
