What was found
HYDRAULIC 3 LOW PRESS CAUTION AT CRUISE WITH INBD GROUND SPOILERS FAULT CAUTION. HYDRAULIC QTY SHOWED 0%. DECLARED EMERGENCY AND RETURNED TO DTW. HAD TO PERFORM EMERGENCY EXTENSION OF THE LANDING GEAR. FLIGHT 3558 - FILED CLOSEDTHIS DISCREPANCY WAS TRANSFERRED TO CAMP ITEM 20490627 WITH THE FOLLOWING CORRECTIVE ACTION: "REMOVED AND REPLACED #3 HYDRAULIC SYSTEM PRESSURE MANIFOLD PRESSURE INDICATOR O-RINGS IN REF TO CRJ900 CMM. OPS AND LEAK CHECK GOOD. ALSO SERVICED HYDRAULIC SYSTEM #3 TO 55% PRESSURIZED IN REFERENCE TO CRJ 900 AMM. HYDRAULICS HAVE BEEN STABLE AT 55% WHILE PRESSURIZED ALL NIGHT "