MJB the new top of the line Exmarks are using the 21 cc Hustler pumps? the problem with the Hustler was they were asking to much from lower end hydro components.
If they would have used them to do a 12 or 13 MPH setup I think the issues would have never been their. The new system of course everybody knows is in a class by itself.
The problem with the all in one newer hydro drive units that ZTR companies are using is you need to keep the fluid fresh because they get dirtier quicker because the capacity of oil is so small.