Sounds like your doing exactly as Grasshopper spells out in the owner's manual. 1,300 hours is not bad hours on the machine I have seen units with over 2,000 hours still working perfectly. I did have a 1982 1600 Grasshopper tha did get a bit sluggish on the right side around 1994 back in "82" we didn't have all the super oil but by the 90's Grasshopper had come up with the new hydro fluid which I put into the hydro's, Wow made a new machine out of it. I traded a year later but it was still working well.
As far as your problem it would lead you to believe the pump or motors had a problem, I have not heard about anything. Ouch the replacements are costly!
Good Luck,