Even if a piston seal was leaking... it wouldn't curl on it's own if the spool valve were functioning properly. I agree with JJ... the problem is in the spool valve.
Mechanos! I did not say J_J was wrong. I said that leaking piston seals can cause drift even if spool valve is perfectly centered.
I think this is one of the most common myth's in hydraulics.
Mechanos! Study my drawings below.
The cylinder can extend from gravity if piston seal leak, the only pressure that will hold the piston (rod) back is the atmospheric pressure.
Volume on rod side of piston is less than the capped side....This is a very common effect from leaking piston seals.
If gravity would push the piston in, retract there will be pressure intensification, cylinder piston (rod) will hold up until pressure open the work port relief or break something...
Like jinman says, it is also important to take into account how fast it leaks down too....but still, most FEL's curl down on cylinder extend,
and it ends up to be the "wear part" in the system that have failed...the piston seal.
This bucket curl down on cylinder retract.
Another possibility is if there is an inline regen valve involved
See the difference between the two cases on these two pictures