Clutch Master Cylinder Leaking again ! Fiasco

You could be right about that. I never noticed any sort of hard coating inside the slave cylinder though the piston does seem to be coated in something.

I always surmised that the issue was a sort of solvothermic reaction between the powder forged aluminum and mineral oil. These two materials are known to not get along well when the fluid gets warm. As the bore eroded, it would enlarge allowing the piston to get cock-eyed and bind up when the thing got hot. It would either slow leak or the piston would bind and get cracked up by the pressure exerted trying to engage the clutch. For me the swap to a synthetic fork oil make all the difference as no longer do I see any black (aluminum oxide) suspended in the old oil when I do my annual swap out and have had no issues.

As I remember, master pump and slave have their own hydraulic circuit and do not exchange liquids. Correct?
If that so, no problem in using other (fork oil) than clutch fluid (magura blood).

Am I making any sense?

I think it's one circuit.

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The two cylinders communicate, as in school physics 5th grade.

The slave cylinder is coated inside out. I damaged one so I know.
Cheers.

May have been a change then as my original '06 slave and a spare I bought back then are both manufactured with simple machined bores.