AN ATR42 IN CRUISE AT 14000 FT DECLARED AN EMER AND DIVERTED DUE TO LOSS OF THE GREEN HYD SYS PRESSURE AND QUANTITY CAUSED BY A FAILED FITTING OR LINE.

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WHILE AT ALT ENRTE FROM ORD TO LACROSSE; WI; A COCKPIT PANEL LIGHT ILLUMINATED INDICATING HYD FLUID 'LOW LEVEL.' THE PNF (THE CAPT) RAN THROUGH THE APPROPRIATE ABNORMAL CHKLIST WHICH; AMONG OTHER THINGS; REQUIRED THE SHUTOFF OF ONE OF OUR HYD PUMPS. THE CHKLIST STATED THAT AS A RESULT WE WOULD LOSE THE ABILITY TO NORMALLY EXTEND THE LNDG GEAR AND THAT WE WOULD LOSE ALL NORMAL BRAKING. THE PNF INFORMED COMPANY OF OUR PROB WHILE PF (MYSELF) TURNED THE ACFT AROUND BACK TO ORD AND INFORMED ATC OF OUR PROB. WE WERE CLRED BY RADAR VECTORS TO ORD; HAD TO EXECUTE AN EMER GEAR EXTENSION PROC (GRAVITY GEAR EXTENSION); LANDED ON RWY 27L; SLOWED USING REVERSE THRUST ONLY; AND FINALLY CAME TO A STOP WITH THE EMER/PARKING BRAKE ON ONE OF THE 'HIGH SPD' TXWYS. SINCE WE COULD NOT TAXI WITHOUT BRAKES; PAX AND BAGS WERE UNLOADED BY OUR COMPANY OUT ON THE TXWY. THERE WERE NO PAX OR CREW INJURIES AND NO DAMAGE TO ACFT EXCEPT LEAKING HYD FLUID FROM THE BELLY OF THE ACFT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE EXACT CAUSE OF THE HYD FLUID LOSS IS UNKNOWN AS MAINT ONLY ADVISED THE CREW A LINE OR A FITTING ON THE LINE FAILED. THE RPTR SAID THE SEGMENT OF LINE AND FITTING LEAKING WERE IN THE GREEN SYS LOCATED IN THE WHEEL WELL.