Don't know if this is your problem, but is my pet peeve on newer tractors with pressure compensated hydraulics that pump at 2900 psi if you are trying to get more oil thru the motor then it needs. They will ruin hydraulic motors. I have put pressure gauges in line where the pressure line goes into motor so I can see how many psi is going into motor. These motors are only designed for 2000 psi or less.  I have ruined more then 1 motor like you have because of this.