Currently have Kubota L4600HST, Kubota M7040, Kubota M8540, two Massey 265's, Massey 230, ...... Had Long 510, Mahindra 3510.Ford 4000, Deere 4230.
if you put bucket on ground and boom down will it roll back? Sounds like blown packing in the cylinders.
if the QD are the same you could also try putting the bucket retract hose onto the boom lift QD and see if it works that way. if it does it isnt the packing but thats the most likely culprit.
/ 1996 kubota 3600 diesel: bucket stuck in dump position