That could be something as simple as an external blockage on the joystick linkage, or much more complex - that loader valve spool has dual poppet type check valves, an adjustable spring-loaded float with (IIRC) 3 small ball bearings that snap into a detent groove, and multiple o rings - I've worked on everything from hot rods/dragsters to heavy equipment (my own), industrial process automation systems, etc, and after seeing what's inside that valve I wouldn't attempt to take it apart without a manual. Too many "surprises" inside
I have two, the original Case paper manual ('bout 5" thick) and the 580B version from here
CASE 580CK C Loader SERVICE Manual PTS -2- MANUALS 580C for sale
The paper ones are expensive, bulky, and not as friendly - only reason I have one is because (at the time) that was all you could get. IIRC, it cost around $150.
No complaints about the one I linked to, you can get just the service manual but the parts manual is useful too.
Not knowing what your background/experience is, hopefully I'm not "talking down" to you, not my intent - personally I'd rather be warned about something like this than to find out the hard way what's in Pandora's box
If you're desperate and can't wait for a manual, I might be able to post a couple of pages from mine - let me know what you want to do... Steve