A B737-200 HAS ONE OF ITS HYD SYS FAIL IN ZJX AIRSPACE. FLC DECLARES AN EMER AND RETURNS TO DEP ARPT.

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DURING CLBOUT; WE NOTICED A SYS HYD QUANTITY AT APPROX 1.3 GALS DECREASING. WE CONTACTED OUR MAINT COORDINATOR AS THE QUANTITY CONTINUED TO DECREASE TOWARD ZERO. BOTH THE HYD 'A' AND 'B' PRESSURE INDICATORS STARTED FLUCTUATING. THE CAPT DETERMINED A TOTAL A/B SYS FAILURE WAS IMMINENT AND MENTIONED THIS TO MAINT COORDINATOR. MAINT SUGGESTED TURNING OFF B-PUMPS TO POSSIBLY SALVAGE HYD FLUID. THIS WAS ACCOMPLISHED; AN EMER WAS DECLARED; AND CHKLISTS FOR LOSS OF A/B SYS (MANUAL REVERSION) WHILE RETURNING BACK TO THE DEP FIELD. ALL CHKLISTS WERE COMPLETED AND AN UNEVENTFUL LNDG; STRAIGHT AHEAD STOP ON RWY; AND TUG BACK TO GATE WERE ACCOMPLISHED. UPON INSPECTION; LCL MAINT FOUND 'A' SYS WAS THE CULPRIT WHILE 'B' SYS WAS STILL OPERATIONAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THERE WAS NOT TOO MUCH DIFFERENCE IN FLYING THE ACFT EVEN THOUGH THE FLT CTLS WENT TO MANUAL REVERSION. IT TOOK A LITTLE MORE THAN NORMAL CTL FORCE TO MOVE THE CTLS. MANUAL GEAR EXTENSION WAS SIMPLE ENOUGH THAT THEY HAD AMPLE TIME TO THINK ABOUT THEIR PROBS. THE 'A' SYS HAD FAILED BUT; THE LOWER CAPACITY 'B' SYS WORKED WELL. THEY HAD TO BE TOWED OFF OF THE RWY UPON LNDG AS THEY LOST THEIR NOSEWHEEL STEERING WITH THE LOSS OF THE 'A' HYD SYS.