Stripped the MC before breakfast. Blimey, no more than about 15 mins from bonnet up. Nothing difficult about that at all. There was a lot of crap in there but in truth the seals looked fine. The bore had some very minor marks but nothing really obvious. Going to replace it anyway as I have sufficient evidence to suggest that fluid may well be creeping past one of the seals at least. Operating it by hand is one thing but when it's full of fluid and under real pressure, it's a different story. Of course it could have been a bit of crud washing around in there causing the problem. I'll probably rebuild it as a spare to go on the shelf with the rest of the salvaged bits.
Chris