"Open center" oil from pump passes through the valve assm. back to tank when valves are in neutral position.
"Closed center" oil from pump is dead-ended at the assm. when valves are in neutral.
Some Assemblies can be converted to do both.
The creeping loader can be caused by several things. Leaking cylinder( internal or external leaks)and yes, worn valves.
Be careful when buying a replacement valve assembly. There are many things to consider:
-load check valves