Hydro pump damages because air was in the line causing this pump to get hot and the head got hot and caused a bur , the plate got hot then a bur formed and cut a groove in the thing that holds the pistons, the plate has to sit FLAT on top of the piston kit.
The oil must of been low enough and air in the line.
Most likely caused by the last time the hydro oil was changed and did not get all the air out and then check fluid level, the Hustler mini z only has 230 hours on it, the oil filter was loose and sucked in air.
Make sure to check your hydro oil on flat ground and add oil untill it is over top the inlet holes inside the tank.