B767 FLT CREW REPORTS CENTER HYDRAULIC LEAK IN THE STABILIZER TRIM LINE. THE PRESSURE LOSS REQUIRED ALTERNATE METHODS OF FLAP AND GEAR EXTENSION AT DIVERT ARPT.

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DURING TAXI OUT THE FO POINTED OUT THAT THE CTR HYD SYS QUANTITY WAS NEAR RF LEVEL. SINCE IT WAS WITHIN LIMIT WE CONTINUED AND TOOK OFF. AFTER REACHING CLB WE CHKED THE LEVEL AGAIN AND IT WAS AT 90 WHICH WAS A GOOD BIT MORE THAN WE HAD READ DURING TAXI. WE CONTINUED TO MONITOR THE CTR HYD QUANTITY BUT NOTICED AS WE NEARED THE EAST COAST OF CANADA PRIOR TO ENTERING ATLANTIC ETOPS THAT THE LEVEL WAS DROPPING. WE NOTED THAT IT WAS DECREASING AT A RATE OF ABOUT .01 PER 7 MINS OF FLYING. BY THIS TIME THE LEVEL HAD DROPPED TO BELOW RF AND CONTINUED TO DROP. I FIRST SENT AN ACARS NOTE TO ZZZZ MAINT ABOUT THE OBSERVATION AND ASKED FOR AN OPINION. I GOT NO RESPONSE FROM THEM. ONCE WE CONFIRMED THE LEVEL CONTINUED TO DROP I MADE A SATCOM CALL TO DISPATCH AND HAD MAINT CTL PATCH ON. WE HAD A DISCUSSION AS TO HOW TO PROCEED BUT I CONTINUED TO SEE THE QUANTITY DROP AND QUICKLY MADE A DECISION TO RETURN TO THE UNITED STATES. WE FOLLOWED THE FLT MANUAL PROC FOR C HYD QUANTITY EICAS EVEN THOUGH WE HAD NOT REACHED THE .50 LEVEL. I SAW THAT WITHIN TIME WE WOULD REACH THAT LEVEL ANYWAY. THE PROC LED US TO TURN OFF THE CTR PUMPS TO PRESERVE WHAT FLUID WE HAD AND TURN THEM BACK ON DURING APCH TO LAND. I MADE A DECISION TO DIVERT TO ZZZ AS IT IS A MAINT STATION AND SEEMED CLOSEST ARPT FOR US WITH GOOD FACILITIES. IT WAS ABOUT 1 HR 30 MINS TO ZZZ. DURING THIS TIME WE BRIEFED THE CABIN PURSER AND HAD THEM USE A CABIN ADVISORY. WE ALSO REVIEWED THE CHKLIST FOR C HYD PRESSURE LIGHTS IN CASE WE GOT INTO THAT SIT. AT THIS POINT I WAS A BIT DISAPPOINTED WITH OUR MAINT DEPT BECAUSE WE TRIED TO REACH THEM TO ASK AT WHAT LEVEL OF QUANTITY WOULD WE EXPECT TO LOSE PRESSURE AND BE IN THAT SIT. MAINT CTL COULD NOT CONNECT US TO THE B767 CTLR AND ONLY A B737 CTLR COULD SPEAK TO US SO WE DID NOT GET GOOD INFO BACK. AS WE DSNDED INTO ZZZ AT ABOUT FL200 AS THE FO FOLLOWED THE APCH DSCNT CHK WE THEN GOT THE C HYD PRESSURE EICAS MESSAGE. WE IMMEDIATELY DECLARED AN EMER; ASKED FOR EMER EQUIP IN ZZZ AND TO LAND INTO THE WIND WITH THE LONGEST RWY. I HAD THE FO FLY WHILE I RAN THE C HYD PRESSURE LOSS CHKLIST UP TO THE POINT OF PUTTING THE GEAR DOWN. I THEN TOOK COMMAND OF FLYING AND HAD THE OTHER 2 FO'S FINISH THE LIST. IT WORKED BETTER THAT WAY TO FLY THE APCH MORE NORMALLY. WE HAD TO LOWER THE GEAR WITH ALTERNATE METHOD; LOWER THE FLAPS WITH ALTERNATE FLAPS SWITCHES AND LAND FLAPS 20 DEGS. THE LNDG PROGRESSED NORMALLY AND WE WERE ABLE TO EXIT THE RWY AT THE END AND TAXI UNDER OUR OWN CTL AT THE GATE. ZZZ MAINT FOUND A PIN HOLE HAD RUPTURED THE HYD LINE FROM THE STABILIZER TRIM. IT WAS REPAIRED IN ZZZ AND WE FERRIED THE ACFT BACK TO ZZZ1 THE NEXT DAY.