What was found
AIR TURN BACK FOR LOW PRESSURE HYD LIGHT ILLUMINATED AFTER ROTATION. PRESSURE DROPPED TO 1600 PSI. LIGHT EXTINGUISHED AND PRESSURE RETURN 2900 PSI. AFTER GEAR UP AND LOCK. GROUNDED - YES. AIR TURNBACK - INSPECTED HYDRAULIC SYSTEM FOUND NO DEFECTS. SUSPECTED AIR IN THE HYDRAULIC SYSTEM. BLED THE HYDRAULIC SYSTEM IN REF B737-900 AMM 29-00-00-870-801. CHECKED FOR HYDRAULIC EXTERNAL LEAKAGE IN REF AMM 29-00-00-790-801. LEAK CHECK GOOD. OPERATIONAL TEST OF THE HYDRAULIC SYSTEM A AND B GOOD PER AMM 29-11-00-700-801. OK FOR SVC.