I have had the same problems with the 25mm cylinders. In the end I gave in to the idea of rebuilding mine. It wasn't that simple though. My local Ford dealer insisted that the rebuild kit was no longer available. I didn't believe them, so I phoned again a few days later and spoke to someone else. He said the same. This happened 3 times in total. Then I spoke to a friend who works for Ford and he managed to sort one out, so they are still available and thats what we used.

Shame I didn't keep a note of the finis code though.