I think your clutch problem is very intriguing. I never heard about that.
But a case is only lost when we give it up.
And we don't want to, are we.
So I decided to do a "brainstorming" about this with my Indy. He is a honest and modest AND independant MB specialist, ashamed to ask the prices he has just to survive.
MB-stealership in my home town goes to him to get all those 6xx rebuild ( and they have a famous MB-rebuilder just 15 miles away in the next town !).
Well, I'll try to explain what reasonable explanation he came up.
As you allready understood : English isn't my mother's language so I am sorry and please feel free to ask any questions. So here we go.
Neither of us believe your theory about the hydraulic system. Everything you told is correct but again neither of us could imagine that could cause enough force to make the clutch slipping.
You replaced the clutch by a new one, excluding the clutch as a cause.
But did you checked out the wear (the real thickness of the flywheel at the contact-with-the-clutch surface ? The wear of the plate pressing down the clutch to the flywheel ( don't know the correct name in your language ) ?
If you did't : do it.
If the contact surface on the flywheel is still OK ( or even when it's not ),
let eventually grind (mill ?) down the surface on the RIM of the flywheel, there where you bolt on the "pressing" assembly to compensate the weardown of the contact surface and you"ll see : no more slipping when hot !
What do you think of it ?
Don't forget to communicate us a follow up !
Thanks in advance and good luck,