Appears to me the oem keeper has been removed in the past.
What is retaining the piston is simply corrosion/galling of the outside section of the bore. Press the piston forward with a punch, use emery paper to polish the corrosion off the bore and then remove the piston.
You must keep in mind...these girling master/slave combos are not precision items and are fairly crude in construction. But if assembled using a modern rebuild set, they will use plain old dot 3 fluid and last ok. There is no reason to use the proprietary girling fluid if the cylinders have been rebuilt using common us-supply parts.