Hello,  That is sure one interesting air motor that Randolph Herr
acquired.  I have no familiarity with it specifically but I did
discover a secret for disassembling pot metal parts.  It may be that
there is an inside fit into the cylinders from the end plates and
that this is sticking.

Pot metal some times grows to form a tight fit between parts that were
not originally joined.  I have tried various methods to restore pot
metal Play-a-Sax toys and finally discovered a solution that works
remarkably well.  I had tried all sorts of penetrating oils without
success before trying one called Kroil (you can find it online) which
does not seem much different from other products but it seems to have
the ability to free very sticky pot metal parts without damage.

To use it on pot metal, carefully introduce it to the joints if the
surfaces are painted and are not to be damaged, otherwise, just flood
it in.  Do this several times over a period of days, if possible and
warm gently with a heat gun if this will not damage nearby parts.
Seized parts that would ordinarily have to be cut apart may slip apart
with little effort.

Best regards,
Spencer Chase