What was found
AFTER TAKEOFF, LT MLG UNSAFE (RED) LIGHT ILLUMINATED BOTH PRIMARY AND SECONDARY. PERFORMED APPROPRIATE CHECK LIST. DUMPED FUEL,APPROX 20,380 GALLONS, RETURNED TO AIRFIELD WITH UNEVENTFUL LANDING. ACFT RETURNED TO GATE. MX PERFOMED INSP WHEN ACFT RETURNED TO GATE, FOUND LT MLG TRIM CYLINDER LEAKING (FAILED). R & R TRIM CYLINDER IAW32-37-01, OP'S AND LEAK CHECKED SATISFACTORY. ACFT RETURNED TO SERVICE.