Hello Paul,
I am not familiar with the servo you are working on so I'm guessing at what your problem is.
If you are having trouble getting a piston with a rubber seal into a bore and it is being dislodged from its location on the piston I suggest that you use something like a hose clip to squeeze it down then tap the piston gently into the bore with a suitable lubricant (brake fluid?) The same principle as a piston ring clamp.
Alec