What was found
AIRCRAFT WAS GROUNDED. CUN, AIR INTERRUPT, MIA-MEX. EMERGENCY DECLARED. LANDING UNEVENTFUL. DURING CRUISE, RECEIVED YELLOW HYDRAULIC SYSTEM LOW PRESSURE WARNING ACCOMPANIED BY FLUID LOSS. MAINTENANCE FOUND INTERNAL -O- RING, ON THE YELLOW RESERVOIR DRAIN VALVE MISSING. REMOVED AND REPLACED DRAIN VALVE IAW AMM 29-13-41. OPERATIONAL AND LEAK CHECK NORMAL.