Today I spent more time investigating the problem. First I checked that the pump is delivering oil. It works fine. Then I refitted the replacement cylinder which I got from a friend, having cleaned it and checked that the valve mechanism isn't seized. Then I started the digger, lifted the front wheels and cranked the steering back and forth several times. When I lowered the front wheels the steering still didn't work, so I topped up the oil. Then I lifted the front end and cranked the steering through several more full cycles (must have been about 10 in total). The steering still didn't work, although I did get the impression that there might be a bit of steering force now and then. But I couldn't really tell. It might just have been that I hauled the wheel especially hard, or that one wheel hit a rut, or something. I managed to transplant a small tree with the machine even though the steering isn't working. Every time I needed to turn I had to lift the front wheels, but fortunately this is easy on a digger. I really want this fixed, though, and I'm tempted to start calling spares places looking for a replacement cylinder. So far I've reached the following conclusions:
(i) the pump is fine (ii) the oil level is fine (iii) there isn't an airlock
So the problem must be either:
(i) the piston seal, or (ii) the valve mechanism
The shaft seal on the cylinder doesn't leak. I would have thought that if the piston seal was leaking, there would still be quite a noticeable steering force present. But I can't really see what can be wrong with the valve mechanism, except possibly a broken spring. The mechanism isn't seized and returns to the central position when the steering forced is removed. Looking into the ports of the original and replacement cylinders, the valve mechanisms do look slightly different (different spool positions), but nothing looks obviously broken. Any ideas what the likely culprit is?
Tom, if you do get chance to scan the exploded diagram I'd be most grateful. At the moment the workings of the valve mechanism are a bit of a mystery!
As I said, I'm tempting to start looking for a replacement. As I have two cylinders, I should theoretically be able to make a working cylinder, but it's a pig of a job. The valve mechanism needs special tools to disassemble. I guess I could make the tools but I'm not sure it's worth it.
Thanks for all the help.