Hello Jim,
This is a long shot but I will relay the story to you in case it can be a help.
Years ago (about 25) my brother and I switched the brake and clutch fluid in his RX7 to Silicone (a very bad mistake). For a brief moment in time many people felt this would be an improvement because silicone fluid wont absorb moisture. The unfortunate side note is moisture is heavier than silicone brake fluid and will sink to the low spot in the system. When using the brakes in a performance way, the moisture against the puck would quickly boil and the brake fade was extreme and dangerous.
So back to the possible relevant part for you. When we flushed the silicone out to go back to a quality performance brake fluid the clutch cylinder started acting just as you described in the Sprinter. We could not get the clutch master to seal with normal break fluid. Everything else worked fine but the clutch master would not. We finally threw in the towel and went back to Silicone for just the clutch fluid. It immediately sealed and started working fine. I wonder if you could have accidentally used Silicone break fluid and given yourself a problem.
Good luck,
Nolan