As an update to this: The problem has been solved!
Resetting the CCT seems to have fixed it. It's a very easy procedure, when you know how. Mildly frustrating when you don't have a clue how it all fits together. Service manual was handy here
I did not have the proper bit to hold the piston in place while I re-installed the CCT, so I ended up using a bent piece of metal which shot off violently a few times while trying to get it in place. Fun times.