EA30 UPON NOTING SLOW LOSS OF HYD FLUID ELECTS TO RETURN TO MIA FOR PRECAUTIONARY EMER LNDG.

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LOCATION: MIA. AFTER DEPARTING MIA AT CRUISE ALT (FL290) I WAS DOING MY NORMAL CHKS OF SYS AND DISCOVERED THAT OUR GREEN HYD FLUID RESERVOIR WAS LOW (BOTTOM OF THE GREEN ARC) AT WHICH TIME I CONTINUED TO MONITOR THE SIT. AFTER 45 MINS; THE LOSS OF FLUID WAS VERY SLOW BUT WAS LOSING FLUID. AT THIS RATE IT WAS DETERMINED THAT WE WOULD BE BELOW THE RED DOT IN THE RESERVOIR AND WOULD HAVE TO LOWER THE LNDG GEAR MANUALLY AT DEST IN WHICH TIME THE PARTS NEED TO REPAIR THE ACFT WOULD NOT BE AVAILABLE. THIS ACFT IS SCHEDULED TO TURN WITH MINIMUM TIME ON THE GND IN LIMA. IN ADDITION; I WAS WORRIED THAT IF WE ENCOUNTERED MORE PROBS WE WOULD HAVE TO LAND AT AN OFF STATION ENRTE. THROUGH FLT SUPPORT LIMA HF RADIO (PHONE PATCH); I CONTACTED MAINT AND DISPATCH TO DISCUSS THE SIT AT HAND; AT WHICH TIME I DECIDED TO RETURN TO MIAMI AND REPAIR OR CHANGE THE ACFT. WE CONTACTED KINGSTON CTL TO REVERSE OUR COURSE FOR THE RETURN LEG TO MIAMI. AFTER 1 HR AND 35 MINS OF FLT; WE TURNED BACK TO MIAMI. AT THAT TIME; TO MAINTAIN THE REMAINING FLUID IN THE GREEN HYD RESERVOIR; I DEPRESSURIZED THE GREEN SYS. SLOWING TO .65 MACH; WE DSNDED TO FL230 FROM FL310. AT THIS POINT; I BRIEFED THE PURSER OF OUR SIT AND I WOULD KEEP HER UPDATED. BTWN THE 2 OF US; WE TOLD THE PAX OF WHAT HAD TAKEN PLACE AND WHAT TO EXPECT. SEEING WE WERE GOING TO BE OVERWT FOR OUR LNDG (321000 LBS) I CONTACTED DFW APCH AND ADVISED THEM OF THAT FACT. DISPATCHER SAID HE WAS GOING TO DECLARE AN EMER. ENTERING MIAMI AIRSPACE; CTR QUIZZED US ON OUR EMER SIT. AT WHICH TIME I INFORMED THEM OF OUR PENDING OVERWT LNDG. AT 35 MI FROM THE MIA ARPT; I PRESSURIZE THE GREEN HYD SYS. WATCHING VERY CAREFULLY THAT WE DID NOT LOSE THE REMAINING FLUID; WE FINISHED THE REMAINING APCH UNEVENTFULLY. LNDG WT WAS 316000 LBS AT A SINK RATE OF 25 FPM AND A VERY SOFT LNDG. MY OVERWT LNDG WAS MADE USING THE CAPT'S EMER AUTH BASED ON A CONDITION OR COMBINATION OF CONDITIONS EXISTED WHERE AN EXPEDITIOUS LNDG COULD REDUCE THE EXPOSURE TO THESE OR ADDITIONAL PROBS WHICH MIGHT RESULT IN A COMPROMISE OR DEGRADATION OF SAFETY. I WANT TO THANK EVERYONE THAT TOOK PART IN THIS OP. IT TOOK A LOAD OFF MY MIND THAT EVERYONE PULLED TOGETHER ON THIS EMER.