What was found
AFTER TAKEOFF AND GEAR RETRACTED, GEAR UNSAFE LIGHT WAS ILLUMINATED. RECYCLED GEAR WITH SAME RESULTS. PERFORMED GEAR UPLATCH CHECK AND LEFT AND RIGHT MAIN GEAR AND DOORS, 'NOT LATCHED' LIGHT WAS ILLUMINATED. REPLACED SYSTEM SELECTOR VALVE FOR BYPASSING. OPS CHECKED NORMAL IAW DC8 MM 32-36-00.