What was found
THE LT HYDRAULIC SYSTEM LOST ALL FLUID WHILE AT 26,000 FEET. THE LOSS OF FLUID WAS ACCOMPANIED BY ILLUMINATION OF THE LEFT HYDRAULIC LOW PRESSURE LIGHT. THE AIRCRAFT WAS RETURNED TO IAH WHERE IT LANDED WITHOUT INCIDENT. MAINTENANCE FOUND B-NUT LOOSE ON LEFT CASE DRAIN FILTER. THE B-NUT WAS SECURED AND THE HYDRAULIC SYSTEM WAS SERVICED. AN ENGINE RUN WAS PERFORMED AND NO LEAKS WERE NOTED.