It's probably a cylinder seal leaking fluid past it. Not seeing leaked fluid outside a cylinder doesn't mean all is OK. I know just one person that tried fixing a leaking trim piston, ordered the wrong kit, lost lots of fishing days, then turned it over to a marine repair shop a month later. They have a special piston seal tool, and should be able to recondition major parts. The seals inside the pistons are replaced.  In short, I think it's a bad idea to try simply because you are asking, which suggests you are not prepared to do it.

Jim

I have to agree that hydraulic cylinder repair is not a DIY kind of project. There are many special tools required and special procedures that must be followed. Better to have the job done proffesionally. Tom