AN MD88 RETURNED TO THE FIELD DUE TO UNABLE TO RETRACT THE LNDG GEAR. FOUND THE MAIN GEAR HYD BYPASS VALVE LEVER NOT IN STOWED POS.

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GEAR FAILED TO RETRACT AFTER TKOF XA34. ZZZ TWR CONFIRMED ALL LNDG GEAR DOWN. RETURNED TO ZZZ UNEVENTFULLY. DISCOVERED 'MAIN LNDG GEAR BYPASS LEVER' WAS NOT IN THE STOWED POS. THE EARLY MORNING FLT OUT OF ZZZ FOR THIS ACFT HAD BEEN MAINT CANCELED DUE TO A HYD LEAK AND FAILURE OF A HYD CHK. A PUMP AND VALVE FOR THE PTU WERE ORDERED FROM ANOTHER STATION; HOWEVER; THE VALVE WAS NOT SENT AND THE WRONG PUMP WAS DELIVERED. CONTRACT MAINT DISCOVERED THE LEAK WAS IN THE AFT STAIR ACCUMULATOR AND DEFERRED THE AFT STAIR AND ANNOTATED THE LEAK AREA HAD BEEN WIPED CLEAN. THEY RPTED THE HYD CHK WAS NORMAL. DUE TO THE ABOVE; ON MY WALKAROUND; I WAS CONCERNED FOR THE GEAR PINS; THE MAIN LNDG GEAR BYPASS LEVER AND THE HYD LEAK AREA. ALL CHKED OK. AFTER ALL INTERIOR AND EXTERIOR CHKS WERE COMPLETED AND WHILE WAITING FOR A NEW RELEASE; WE HAD MORE FUEL ON BOARD THAN SCHEDULED. A FUEL PERSON CAME ONTO THE FLT DECK AND TOLD US HE WAS READY TO DE-FUEL. THE CAPT ADVISED WE DID NOT REQUEST DE-FUELING; ONLY A NEW RELEASE TO REFLECT THE CORRECT FUEL ON BOARD. AFTER THE RETURN TO BLOCKS FOR THE GEAR; WE TALKED TO MR X FOR CONTRACT MAINT. HE CONFIRMED WITH THE INDIVIDUALS WHO PERFORMED THE HYD MAINT THAT THE MAIN LNDG GEAR BYPASS LEVER WAS STOWED WHEN THEY COMPLETED THEIR WORK. HE ALSO ADVISED US THAT FUELING PERSONNEL HAD A HISTORY OF OPENING GEAR DOORS FOR PUMP AND LEVEL CHKS WITHOUT INCLUDING OPS AS REQUIRED AND FURTHER THAT THEY WOULD MANUALLY CLOSE THE GEAR DOORS WITHOUT TELLING ANYONE THE DOORS HAD BEEN OPENED. THE MOST RECENT AT ZZZ WAS AN ACR X B767 ON MAR/SAT/05. I SUSPECT THE FUELING PERSONNEL OPENED THE MAIN LNDG GEAR BYPASS LEVER AS PREPARATION FOR DE-FUELING WITHOUT ADVISING ANYONE.