We have a 2012 Volvo MC135 and the boom leaks down, pulled the lines off the lift cylinder and capped them and the boom doesn't leak down then so it must be in the valving. Anyone come across this before?
The problem turned out to be the load recovery ball valve (near the hydraulic tank) is tied into the lift side of the boom cylinder and it was leaking internally.
Looks like you already answered your question, but all of the JCB skid loaders (and thus the Volvo branded JCB skid loaders) are known to have the boom and bucket settle due to internal leakage in the valve. It is a slow leak - boom maybe drops 1 foot overnight for me. Someone has created a load-check kit that can be added on to stop any settling, but I believe that affects the float function.
That is interesting but this machine has never had the boom settle at all until the last 3 months. When we capped the line to the load recovery ball valve I had the boom cylinder an inch up from resting on the safety stop and 24 hours later it was still an inch up...... that's how I found it was inside that little ball valve.