How much fluid do you lose before the hydraulics stop working? Or do they not work at all immediately?
One other leak point I have seen has been the bleeder hose from the top of HST block to top of the trans case. They are usually gnawed through by mice making nests on top of the trans under the fuel tank. If you go to the link NIW gave,
https://apps.kubotausa.com/illustrated-parts/
search bx24, then select pump/motor system,
then drawing j21002 for hydraulic oil line- delivery. look for item 120
replacement requires the fuel tank to be removed. it makes quite a mess and is hard to pinpoint the source.