AN EMB120 FLC EXPERIENCED A MALFUNCTION OF THEIR GREEN HYD SYS AND CONTINUES ON TO DEST ARPT OF SEA; WA.

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DEPARTED AT XA55. IN FO'S PRE TAXI CHK; SOP IS TO SWITCH GREEN AND BLUE HYD PUMPS TO THE AUTO POS AND NOTE PRESSURE AND QUANTITY. GREEN AND BLUE SYS WERE FULL AND IN THE GREEN RANGE. 15-20 MINS INTO THE 50 MIN FLT CRUISE AT FL180; GREEN HYD FLUID LEVEL WAS NOTED IN YELLOW ARC (VERY LOW LEVEL) FOLLOWED BY A SINGLE CHIME WARNING FLUID LOW LEVEL. GREEN SYS HYD PRESSURE STILL REMAINED IN GREEN RANGE (3000 PSI). QRH PROC LED US TO BELIEVE THAT IF GREEN SYS IS LOST TOTALLY; WE WOULD LOSE OP TO THE LNDG GEAR; NOSEWHEEL STEERING; RUDDER; OUTBOARD BRAKE AND FLAPS. DUE TO THIS; AN EMER WAS DECLARED; DECISION WAS MADE TO CONTINUE TO SEATTLE; BECAUSE SEATTLE WAS THE MOST SUITABLE ARPT IN POINT OF TIME SVCS; RWY LENGTH ETC. DECISION WAS MADE TO FREE-FALL THE LNDG GEAR PER QRH. WE WERE UNABLE TO DETERMINE THE SOURCE OF THE LEAK AND QRH PROC WAS TO NOT LOWER THE GEAR NORMALLY IN THIS SIT. FREE-FALL GEAR EXTENSION WENT NORMAL; 3 GREENS. AT THIS POINT; FLUID WAS VERY LOW; BUT PRESSURE STILL IN GREEN. ON FINAL TO RWY 16L AT SEA; FLAPS WERE SELECTED TO 25 DEGS; CAME DOWN NORMALLY. ON LNDG; FULL REVERSE WAS APPLIED AND SOON DISCOVERED WE HAD NOSEWHEEL AND BRAKING PWR. GREEN SYS PRESSURE STILL REMAINED IN GREEN RANGE. AFTER ENG SHUTDOWN; A LARGE AMOUNT OF HYD FLUID COVERED THE L WHEEL AND ENG COWLING. MAINT LATER DETERMINED THE LEAK WAS DUE TO A BROKEN HYD LINE.