What was found
CLE - EMERGENCY LANDING - FLT 534 - C - EN ROUTE FROM PHX TO CLE RECEIVED YELLOW HYDRAULIC SYSTEM RESERVOIR LOW LEVEL QUANTITY INDICATION WITH PRESSURE INDICATING NORMAL, 1 HR 30 MIN LATER SYSTEM LOST ALL PRESSURE. DECLARED EMERGENCY LANDING INTO CLE, LANDING WAS UNEVENTFUL. MAINTENANCE TROUBLESHOT AND REPLACED DEFECTIVE O-RING ON QUICK DISCONNECT COUPLING AE99116K ON YELLOW SYSTEM HYDRAULIC MANIFOLD. MAINTENANCE ALSO DISCOVERED NR 2 ENGINE HYDRAULIC PUMP CASE DRAIN FILTER DIFFERENTIAL BUTTON POPPED, AS PRECAUTION REPLACED CASE DRAIN FILTER AND NR 2 ENGINE HYDRAULIC PUMP IAW AMM 29-13-51-401.