Since you've had this mower.....did you ever have both of the mowers Hydraulic Pumps serviced?
Fluid changed along with both filters changed as well...etc.?
This is where I would first start to diagnose the problem then proceed from there....it's your call.
If so then start in section 4-1 (as forum member "mechanic mark" kindly provided the link for the Service Manual) thru 4-8 and proceed with that?
If you don't feel like doing the proceed yourself then take your mower to the shop and have them perform the task?
If you do get the problem resolved please post about your success here on the forum and explain exactly what you did to have your mowers problem corrected and running again.
Good luck and I hope you get the problem resolved....and thanks for posting!