Surprisingly the master cylinder for the clutch was in very good condition, not scraped nor corroded and the seals looked fine. The piston came out with a light tap.
However in the slave things were quite different. Using lots of penetrating oil and a vise as a makeshift press I was able to move the piston but it refused to come out completely on its own.
So I decided to pump it out using old engine oil and the working master cylinder. I clamped it to the drill press and slowly filled it with 15W40.
Getting rid of all the air bubbles took a longer time than I expected.
Finally, the slave piston was released. The seal was ok-ish but both the piston and the inner walls had pitting and scratches.
I hung everything upside down and loosened the purging screws to collect the oil. I thought it was a lot more than this but it’s about a small coffee cup.