I noticed that the fluid level in the master clutch cylinder was dropping a little. there was dirt and fluid at the back of the master cylinder, while the slave was clean and dry. Time to change the seal on the master. After removing the master cylinder I could see a proper mess.
After cleaning the master cylinder looked much better. Even the bore showed no scoring or pitting.
Here you see the internal parts and the old seal
The repair kit came with all the new seals I needed.
After reassembly it looked like a "new" clutch master cylinder.
Refitting was easy and after bleeding the clutch worked as good as a new one.