B767-300 FLT CREW NOTICES REDUCTION IN HYD QUANTITY; FLT IS DIVERTED WHEN LEAK CANNOT BE CONTAINED.

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ESTABLISHED IN CRUISE; I ELECTED TO RETRIM THE ACFT. BRINGING UP THE STATUS PAGE; I NOTICED THE CTR HYD QUANTITY RF SYMBOL AND A QUANTITY READING OF .55. WE MONITORED THE QUANTITY AND NOTED A .01 DROP EVERY 3/4 MINS. WE ESTABLISHED CONTACT WITH DISPATCH AND MAINT. SINCE WE WERE ONLY 50 MI PAST ORD; WE DECIDED TO DIVERT TO ORD. BEFORE WE TURNED MAINT SUGGESTED WE TRY TURNING OFF THE #1 CTR ELECTRIC PUMP. THIS SEEMED TO STOP THE LEAK. WE DECIDED TO CONTINUE TO BOS AND MONITOR THE READING. AFTER 15 MINS; IT WAS DETERMINED THE LEAK HAD RETURNED AND THE SOURCE WAS NOT THE PUMP. BOS WX WAS HVY RAIN AND 25 KT WINDS. WE TURNED BACK TO ORD. THE FOLLOWING CHKLISTS WERE ACCOMPLISHED OR REVIEWED: LOW HYD QUANTITY; CTR SYS PRESSURE LOSS; ALTERNATE GEAR AND FLAP OP. A CABIN ADVISORY WAS ESTABLISHED AND PA ANNOUNCEMENT MADE. GEAR AND FLAPS 20 DEGS WERE ESTABLISHED EARLY AND THE HYD SYS WORKED NORMALLY ALTHOUGH THE QUANTITY REACHED .12. RESERVE BRAKES AND STEERING SWITCH WAS PUSHED TO ENSURE STEERING. LNDG AND TAXI TO THE GATE WERE NORMAL.