AN ACR MLG'S LNDG GEAR WOULD NOT RETRACT.

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WE HAD A QUICK ACFT CHANGE IN PIT. THE FO PREFLTED THE OUTSIDE WHILE I CHKED THE INSIDE AND DISCUSSED PREVIOUS WRITE-UPS WITH A MECH WHO HAD JUST COMPLETED WORK ON #2 BRAKE; AFTER PREFLT; WE PERFORMED RECEIVING; BEFORE START; AFTER START; AND BEFORE TKOF CHKLISTS. AFTER TKOF; GEAR HANDLE WAS RAISED BUT GEAR DID NOT RETRACT. WE PERFORMED ABNORMAL CHKLIST DURING VECTORS FOR RETURN TO THE ARPT. THE BOOK SAID THAT; IN OUR PRESENT CONDITION (GEAR HANDLE NOW DOWN; 3 GREEN LIGHTS; 1 YELLOW GEAR DOOR OPEN LIGHT ON); IT WAS NECESSARY TO PULL EMER GEAR HANDLE AND LET THE DOORS 'FLY' WITHOUT HYD PRESSURE ON THEM TO LESSEN DAMAGE ON CONTACT WITH THE RWY. NORMAL LNDG WAS ACCOMPLISHED AND MAINT GOT DOORS UP AND PINNED. AT THE GATE; DIRECTOR OF MAINT SAID THAT A HYD CTL IN THE L WHEEL WELL WAS IN BYPASS; CONTRARY TO NORMAL POS. THE FO; WHO IS A VERY CONSCIENTIOUS PERSON; DISTINCTLY REMEMBERED PREFLTING BOTH WHEEL WELLS. IN ADDITION; TAXI OUT WAS COMPLETELY NORMAL (NO YELLOW LIGHTS; STEERING AND BRAKING NORMAL). THE INVESTIGATION WAS CONTINUING AS WE LEFT ON A REPLACEMENT ACFT.