What was found
AIRCRAFT WAS GROUNDED: FLIGHT CREW REPORTED RECEIVING A YELLOW HYDRAULIC RESERVOIR LOW LEVEL ECAM MESSAGE, THE PTU WAS SELECTED OFF WITHIN 30 SECONDS, NR 2 ENGINE DRIVEN PUMP WAS SHUTOFF WITHIN 30 SECS, THEN TURNED ON, ON SHORT FINAL AFTER LANDING, TURNED OFF. FLIGHT CREW DECLARED AN EMERGENCY AND THE ACFT LANDED WITH NO FURTHER INCIDENT. MAINTENANCE FOUND THE RUDDER SERVO CONTROL FILTER LEAKING AND REPLACED THE O-RING IAW AMM 27-24-51-400-002. A SATISFACTORY LEAK AND OPS CHECK WAS ACCOMPLISHED.