What was found
LEFT HYDRAULIC PRESSURE AND QUANTITY WENT TO ZERO. COMPLIED WITH COM PROCEDURES AND THE LEFT QUANTITY RETURNED TO 5 QUARTS BUT PRESSURE REMAINED AT ZERO. FLIGHT RETURNED TO A SAFE UNEVENTFUL LANDING BACK TO DTW. MAINTENANCE FOUND THE RESERVOIR EMPTY AND FOUND A LEAK DUE TO A BAD O-RING ON THE CHECK VALVE AT THE LEFT THRUST REVERSER CONTROL VALVE. THE O-RING AND THE ENGINE DRIVEN HYDRAULIC PUMP WERE REPLACED. ALSO, NEW FILTERS AND CHECK VALVE WERE INSTALLED. AIRCRAFT RETURNED TO SERVICE.