Thanks for the input, guys! I, too, drove my Mark IX for years with no problems with the power steering, but I was running OEM-type bias-ply tires. Then the car went into a 25-year hibernation, so you can imagine what happened to all of the seals, both in the PS gearbox and the pump. The box leaks, and presumably provides no pressure since the car is a bear to turn. The only shop in the U.S. that we’ve identified to rebuild the box won’t do the work if the car is running radials; they state that the increased footprint provided by radial tires increases the load on the PS system and that will cause premature failure. If that were true, I can see their reluctance to issue a warranty, but so far they seem to be the only voice on the planet singing that tune.
Re-sealing the box is probably all that is required, but that thing is complex enough that we prefer to engage the services of a specialist. Too bad Jaguar didn’t use a generic Saginaw box.
I am running big tires on the Mark IX, by the way. I went with a common SUV tire because they’re readily available nationwide, relatively cheap, and they provide a similar circumference to the OEM tires that the car was delivered with. Therefore speedometer readings are accurate and the ground clearance is adequate.
Thanks again for your considered responses, and please let me know if you have any suggestions on sourcing a rebuild for the box.