AN LGT COMMERCIAL FIXED WING ACFT AT TOP OF DSCNT DECLARED AN EMER DUE TO LOSS OF 'B' HYD SYS PRESSURE AND QUANTITY. CAUSE OF HYD LOSS UNKNOWN.

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AT TOP OF DSCNT; SO TURNED ON #1 'B' HYD PUMP (NORMALLY OFF FOR CRUISE). SO STATED THAT 'B' HYD SYS WAS FLUCTUATING; THEN 'B' PRESSURE DROPPED TO ZERO WITH BOTH PUMPS OPERATING. 'B' HYD QUANTITY WAS APPROX 1 GAL; SYS 'A' QUANTITY WAS 2.5 GAL. ACFT CTL WAS XFERRED TO FO. HYD LEAK OR LOSS (HARD CARD) WAS ACCOMPLISHED IMMEDIATELY AND THEN FOLLOWED UP WITH POM HYD LEAK OR LOSS IN MANUAL. IT WAS DETERMINED SYS 'B' LEAK OR LOSS FOLLOW UP WAS REQUIRED IN MANUAL; AND WAS ACCOMPLISHED AS WELL AS REFING ODM FOR FLAP 15 DEG LNDG DISTANCE. FLT ATTENDANTS WERE NOTIFIED OF THE PROB AND ASKED TO PREPARE CABIN FOR ARR. WX AT BUF WAS RPTED TO BE HVY RAINSHOWERS PASSING THE FIELD SO THE CREW ANALYZED THE POSSIBILITY OF DIVERTING TO ROC. WE INFORMED FLT CTL AND MAINT OF PROB AND ASKED FOR ROC WX. WE WERE GIVEN EXTENDED VECTORS TO WORK ON PROB AND ASKED TO HAVE EMER EQUIP AVAILABLE. AFTER ACCOMPLISHING DSCNT AND APCH CHKLIST FROM POM; IT WAS DECIDED THAT WE WOULD LOWER GEAR TO FIND OUT WHAT BRAKE PRESSURE WAS AVAILABLE. THE WX AT BUF WAS RPTED AT 30 MI VISIBILITY; RAIN HAD STOPPED. WHEN LNDG GEAR WAS LOWERED; THERE WAS A +1200 PSI DROP IN HYD BRAKE PRESSURE AS STATED IN BOOK AND PRESSURE STABILIZED AT 1900 PSI. THE BRAKE INTERCONNECT WAS OPENED AND HYD PRESSURE RETURNED TO 3000 PSI. WE DECIDED TO LAND AT BUF. ALL OTHER INDICATIONS APPEARED NORMAL AND THE APCH AND LNDG WAS ACCOMPLISHED AT BUF BY THE FO. LNDG; ROLLOUT AND STOPPING WERE UNEVENTFUL. TAXI TO THE GATE WAS ACCOMPLISHED AFTER NOTIFYING FLT ATTENDANTS AND PAX THAT EVERYTHING WAS OK.