I’m thinking it might be either leaking seal in the cylinder, or leaking
oil, or possibly
water ingress. Air in the system causes uneven movement and ‘bumps’, but usually not a slow fade, except when the
oil level is very low.
First, thoroughly bleed the system to
rule out the simplest fix. Crack your high point reservoir, then turn your
rudder to full lock one way. Open the bypass valve on or near the
steering cylinder and manually move your
rudder back and forth through full range of motion. Close the bypass valve, turn your rudder to full lock the other way, then repeat.
Work better now?
Next check is your oil level - check your header tank - it should be not empty and hydraulic oil should be visible. If not, you may have an
oil leak somewhere in the system. Most likely at the cylinder, but could be anywhere. Is your oil level decreasing slowly? The Platino disaster a few years ago was caused by leaking oil.
If oil level is fine, then it could be a cylinder seal leak. A cylinder seal leak will let it
work normally at first, then it will get worse as oil
leaks from high pressure to low pressure. Try just cracking your bypass valve - does the same thing happen, but faster? If so, check the cylinder shaft
seals - this may need to be done professionally.
Finally, if you’ve removed the cylinder and checked/repaired it, before replacing it drain all the oil out and replace it. Then bleed the system as above.