You can download the workshop manual here. It explains the sytem in considerable detail and is replete with schematics and illustrations:
Your symptoms are a perfect match for power beyond blockage downstream of the loader valve and the typical source of that blockagecwould be a 3pt control valve thats not in neutral:
- The boom exhausts to power beyond (3pt) and the blockage prevents the boom from moving
- The bucket exhausts to tank via tank port on loader valve and it is not blocked.
- The boom spool is serial so when boom and bucket are operating simultaneously boom exhsust oil goes to bucket inlet and from there to tank via tank port so neither function is blocked
I did all of that analysis without changing any parts, spending any money or involving a dealer. All that remains is for you to lower the 3pt and see if it is the source of your problem
Dan