The family of PC-7200 Piston-Cylinders are uniquely designed for high pressure gas operation by sealing the annular gap between the piston and cylinder with a liquid. With the PC-7200 design, the liquid reservoir is internal to the piston-cylinder module. Access to fill and drain it are from the bottom of the assembly via a pair of small passages.
This video shows how to fill / drain the PC-7200 reservoir and it is applicable to all sizes of piston-cylinders.
See this article for part numbers and ordering information for o-rings, tools, lubricants; including the syringe for the above procedure.