"Richard Smith" wrote in message ...

... Modifying the rods into pistons and making packing glands to
retain the seals are lathe operations. ...

I took it a lathe would be available.
Cylindrical "spud", cylindrical bush.

[[[[[[[[[[[[[[[[[
It's difficult to create innovative mechanical solutions with only stock
parts meant to solve standard problems. At Segway which was an engineer's
playground by official policy the CNC lathe and milling machine were almost
always tied up making someone's wild idea. I used their manual lathe or my
home shop machines but my main task was custom electronics.

https://news.yale.edu/2008/12/05/stu...orrell-s-class
Electric model airplanes were practically a second product there, and they
taught me the care and feeding of Lithium batteries.
Unfortunately like bridge building the team dissolved and sought new
challenges once the project was complete.
]]]]]]]]]]]]]]]]]

Maybe the "hydraulic fluid" could be grease - thick - tranmitting to
oil or water through a rubber membrane. So the piston/bush sees
viscous grease not thin oil.

[[[[[[[[[[[[
It's known that high pressure oil seals work if done right, so I'd bin that
with the minor issues that can be solved later by throwing enough money at
them, and concentrate on identifying and resolving the show-stoppers.

I'm glad I read the Amazon reviews before applying the thick black tire bead
sealer goop. A reviewer advised to let it set somewhat before inflating the
tire lest it spray out. I did, and only soapy water rubber lube sprayed me
before the bead seated when I inflated it.

Here is a racing engineer and technology historian who might know about
metal fatigue in oil:
https://www.calum-douglas.com/