Kubota Bucket Won't Curl Down!

My Kubota B3300SU's loader function stopped working properly while my dad was using it. The bucket will not curl down. The control lever is stuck and will not move to the lower position. All of the other loader functions on my Kubota tractor work except for the bucket tilt down function. Why?

The hydrostat/transmission/hydraulic fluid hasn't been changed EVER! I'm going to start there and see how much junk comes out when I drain the trans fluid and see how nasty the filters are. But, I fear the problem is in the spool valve that controls the tilt function on my Kubota tractor.

Help me diagnose this problem, please. Are there any diesel techs or tractor techs that have ran into this issue?

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