B737 ENROUTE EXPERIENCED A HYD FLUID QUANTITY DECREASE; DECLARED AN EMERGENCY AND DIVERTED FOR AN UNEVENTFUL OVERWEIGHT LANDING.

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IT WAS DAY 2 OF 4 AND THE SECOND LEG OF 3. WE SWAPPED ACFT. THE PREVIOUS INBOUND CREW HAD WRITTEN THE ACFT UP FOR A 'FEEL DIF' LIGHT. MAINT WAS PERFORMED ON THE ACFT DRAINING THE ELEVATOR FEEL PITOTS AND THE REPLACEMENT OF A CHK VALVE IN THE HYD SYS. THE ACFT WAS RETURNED TO SVC AND WE DEPARTED. ABOUT 1 HR INTO THE FLT; THE 'FEEL DIF' LIGHT ILLUMINATED. THE CAPT/PLT MONITORING PULLED OUT THE QRH AND WE RAN THE CHKLIST. AFTER RUNNING THE CHKLIST WE OBSERVED THAT THE 'A' SYS HYD QUANTITY WAS INDICATING 66% AND THE 'B' 94%. THE HYD PRESSURES INDICATED NORMAL BUT THE QUANTITIES WERE DROPPING ABOUT 1% EVERY 5 MINS. I WAS THE FO AND PF HAD CONTINUED TO HANDLE THE ACFT AND RADIOS WHILE THE CAPT/PLT MONITORING CONTACTED DISPATCH AND MAINT CTL. A JOINT DECISION WAS MADE BTWN ALL PARTIES THAT THE BEST COURSE OF ACTION WAS TO DIVERT TO ZZZ. WE LANDED AT ZZZ AT 700 LBS OVERWT WITHOUT INCIDENT. AFTER THE CAPT COMPLETED ALL THE WRITE-UPS; WE HANDED THE ACFT OVER TO MAINT. WE WALKED ACROSS THE HALL TO ANOTHER ACFT AND CONTINUED OUR TRIP.