Thanks for the clarification.
If it isn't leaking oil, then there will be an issue with a couple of valves. For the lift cylinder to drift, oil will need to pass SVH and/or CVL, SPL to get back to the tank.
Probably be logical to inspect those valves for debris.
Follow the hydraulic schematic from SVH to the tank.

Lift cylinder drifting down. Unit off.

Need more information.
Assuming the lift cylinder is drifting down, not the tilt drifting down.
Does it still drift with the unit powered off?
You need to determine if the drift is an electrical or mechanical issue by raising the carriage and powering down.
Don't look for a manual release valve as a RC5500 series does not have one.

Have you checked the manual release valve? This is a black **** on the hydraulic block. Typically that is what causes forks to drift down slowly.