What was found
BNA - FLT 937 - AFTER TAKEOFF FROM TYS ON TRAINING FLIGHT, SELECTED GEAR UP HYDRAULIC PRESSURE DROPPED TO ZERO AND RETURNED TO NORMAL. HYDRAULIC PRESSURE STARTED TO FLUCTUATE, CREW SELECTED GEAR DOWN AND NOSE STRUT INDICATED SAFE. CREW PERFORMED MANUAL EXTENSION OF MAIN GEAR. AIRCRAFT RETURNED TO BNA AND MADE UNEVENTFUL LANDING. FOUND NON-RETURN VALVE FOR RIGHT HAD HYDRAULIC PUMP CRACKED, REPLACED VALVE AND PERFORMED FUNCTION CHECK OF LANDING GEAR SYSTEM. AIRCRAFT RETURNED TO SERVICE. (M)