What was found
AFTER REPLACING THE TAIL ROTOR SERVO WITH A TESTED, (TIME CONTINUED), SERVICEABLE UNIT, A MAINTENANCE OPERATIONAL CHECK FLIGHT WAS CONDUCTED. DURING THE MOC FLIGHT, THE UPPER SEAL FOR THE SERVO FAILED, CAUSING LOSS OF NR1 HYDRAULIC PRESS. THE AIRCRAFT RETURNED TO BASE, ANOTHER SERVICEABLE TAIL ROTOR SERVO WAS INSTALLED, MOC FLIGHT CONDUCTED WITHOUT INCIDENT, AND THE AIRCRAFT WAS RETURNED TO SERVICE.