B737 CAPT REPORTS B HYDRAULIC SYSTEM FLUID LOSS DURING CLIMB OUT. FLIGHT CONTINUES TO DESTINATION FOR PARTIAL FLAP LANDING AFTER B SYSTEM FAILS.

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PASSING APPROX FL250 ON CLBOUT; THE MASTER CAUTION LIGHT FLICKERED WITH AN ASSOCIATED 'HYD' INDICATION ON THE PANEL AND A FLICKERING 'B' HYD LOW PRESSURE LIGHT FROM THE ENG DRIVEN PUMP. THE 'B' SYS QUANTITY SHOWED ABOUT 50% FULL; SIGNIFICANTLY LOWER THAN THE 88% FULL IT SHOWED AT PREFLT. SHORTLY AFTER REACHING CRUISE ALT; THE 'B' SYS ENG DRIVEN PUMP LOW PRESSURE LIGHT WAS ON CONTINUOUSLY WITH THE ASSOCIATED MASTER CAUTION LIGHTS. THE FLUID QUANTITY WAS NOW 40% AND SLOWLY DROPPING. WE EXECUTED THE HYD SYS 'B' QUANTITY OIL PRESSURE LOSS CHKLIST FOR A SINGLE (ENG DRIVEN) SYS LOW PRESSURE LIGHT. AT THIS TIME WE CALLED MAINT AND PATCHED WITH DISPATCH. WE DETERMINED THAT IT WAS HIGHLY LIKELY WE WOULD HAVE COMPLETE SYS 'B' FAILURE AS THE FLUID QUANTITY DECREASED. WE DISCUSSED THE VARIOUS EFFECTS OF LOSS OF 'B' SYS AND THE OPTIMUM CONDITIONS FOR RECOVERY WITH THE DEGRADED SYS. THE LOSS OF THIS SYS HAS NO EFFECT ON CTLING THE ACFT EITHER INFLT OR AFTER LNDG; BUT WOULD NECESSITATE USING AN ALTERNATE MEANS TO DEPLOY SLATS AND FLAPS AND WOULD REQUIRE A PARTIAL FLAP LNDG. AFTER CALCULATING THE REQUIRED WT AND RWY CONDITIONS; WE DETERMINED PROCEEDING TO ZZZ WOULD ALLOW US TO BOTH DECREASE SAFELY TO LNDG WT AND GIVE US OPTIMUM LNDG CONDITIONS. MAINT; DISPATCH AND THE FO AND I ALL REVIEWED THE PLAN AND I DECIDED TO PROCEED TO ZZZ AND PREPARE FOR A LNDG WITH FLAPS 15 DEGS PER THE FLT MANUAL 'B' SYS LOSS AND FLAPS (ALTERNATE) OP IRREGULAR PROC. ADDITIONALLY; SINCE WE WERE TO MAKE AN IRREGULAR LNDG CONFIGN APCH AND LNDG; I DECIDED TO DECLARE AN EMER WITH ATC. DISPATCH AND THE FO CONCURRED AND WE MADE THIS CALL TO CTR APPROX 1 HR PRIOR TO LNDG. BY THIS TIME THE 'B' SYS QUANTITY DECREASED TO ABOUT 10% AND BOTH 'B' SYS (ELECTRIC DRIVEN AND ENG DRIVEN) LOW PRESSURE LIGHTS ILLUMINATED. WE COMPLETED THE APPROPRIATE FLT MANUAL CHKLISTS; SECURING THE PUMP SWITCHES. AS WE APCHED ZZZ WE BRIEFED THE FLT ATTENDANTS; REVIEWED THE LNDG AND AFTER LNDG PROCS AND EXPLAINED TO ATC THAT WE WOULD NEED AN EXTENDED FINAL ON THE ILS APCH AND THAT WE WOULD BE ABLE TO TAXI CLR AND PROCEED TO OUR GATE. APCH CTL VECTORED US TO ABOUT A 15 MI FINAL. WHILE THE FO FLEW THE RWY LINE-UP; WE CONFIGURED THE ACFT FOR A 15 DEG FLAP LNDG USING THE FLT MANUAL PROCS. ONCE CONFIGURED; I TOOK CTL; THE FO CONFIRMED THE CHKLISTS COMPLETE; AND I FLEW THE ACFT TO AN UNEVENTFUL LNDG. WE TERMINATED THE EMER AFTER CLRING THE RWY AND TAXIED TO THE GATE. THE FIRE CHIEF; CONCERNED ABOUT POSSIBLE HOT BRAKES; ESCORTED US TO THE GATE. APPROPRIATE AFTER LNDG CHKS WERE COMPLETED AND MAINT AND OPS PERSONNEL WERE BRIEFED.