Does your PRV come on in the full curl down mode? If there is a large leak, the relief will not come on as the fluid can not build up relief pressure. If you put a hyd gage on the lift cyl, and use that pressure, as a reference, you can then put the gage on the curl cyl and compare the pressure.
The cause is probably a leaking seal on the piston in one or both cyl allowing the rod end fluid to flow to the base end. I don't believe a valve spool will leak down that fast, however, if the spring center spool is not in the center, it could pass some fluid to the cyl.
You can check which cyl is leaking by removing, say the left cyl hoses and cap them. Then grab a small load and see if the bucket curls down on it's own. If that cyl holds, reconnect the hoses and check the other cyl by removing the cyl hose and capping the hoses.
If one cyl leaks, some would say repair both cyl, but there is the old saying, that if it ain't broke don't fix it. Then there is that " might as well ". It is your tractor, so it is your choice.
Not to difficult to repair if you can get the correct seal kit.
