Hi,
I put the rebuild kit through the pump and problem solved. I did no other work on the pump. The problem will have been the shaft seal. Interestingly the kit I received contained 3 seals, none of which were the same thickness as the original, but I installed one and it worked.
For anyone using this information in the future, the rebuild is easy. The hardest part is cleaning the exterior. Total time around 6 hours.
Also for future users of this thread, the reservoir on these "pump separate to reservoir" units is most certainly not under vacuum. I can now run the tractor and use the power steering with the lid off the reservoir without any bubbling issues, so there is no vacuum in the reservoir cannister. If you have bubbling, it's not a leaky reservoir cannister.
Thanks for everyone's help.