My go-to cleaning/hydrosonic bath solution is what is spelled out in the 1923 Wahl service manual: 1 part ammonia to 3 partes water (plus a few drops of Dawn).
I ran a plastic Soennecken Schulfüller through the hydrosonic bath after which the piston got well and truly stuck in the fully retractedposition. Drying, soaking, swearing...nothing worked to free it.
Then I remembered this Formula 409 thread. Bingo! Piston was freed up as well as a bunch of gunk. Got it all cleaned out, greased up, and it's good as new!