What was found
AFTER TAKEOFF, LAND GR LVR WAS PLACED IN UP POSITION, FLT DECK CREW REPORTED NO RESPONSE TO GR HNDL (UP) SELECTION, WITH GR REMAINING IN DOWN AND LOCKED POS AND MN GR DOORS OPEN. RECYCLING OF THE GR HNDL WAS NO HELP. MAINT WAS NOTIFIED AND A/C RETURNED. MAINT RECOVERED A/C AND DURING POST FLIGHT INSPECTION MAINT BYPASS DOOR WAS FOUND IN DOWN POSITION. WHEN MAINT BYPASS DOOR WAS CLOSED AND WITH HYDR PRESSURE APPLIED MAIN GR DOORS CLOSED. A/C WAS TOWED TO HANGER, LT GR DOOR WAS REPLACED IAW MM AND A GR SWING WAS PERFORMED IAW MM WITH ALL GR SYS OPERATING NORMAL. BYPASS DOOR LATCH WAS REPLACED DUE TO UNSERVICEABLE SPRING AND WAS DETERMINED TO BE PRIMARY CAUSE OF AIR TURN BACK. A/C WAS RETURNED TO SERVICE.