B737 FIRST OFFICER REPORTS HYDRAULIC SYSTEM A QUANTITY DROPPING IN CRUISE AND PRESSURE DROPPING DURING APPROACH. GEAR IS EXTENDED NORMALLY BUT NOSE WHEEL STEERING FAILS ON ROLL OUT. AIRCRAFT IS TOWED TO GATE.

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I WAS THE PF AND THE CAPT WAS THE PNF. FLT WAS UNEVENTFUL UNTIL WE WERE APCHING ZZZ AT FL360; WHEN I NOTICED THE SYS 'A' HYD SYS WAS READING HALF FULL; AT WHICH TIME I NOTIFIED CAPT OF THE SITUATION. WE WATCHED THE SYS FOR ABOUT 5 MINS AND DID NOT NOTICE A CHANGE AND THOUGHT IT MIGHT BE A GAUGE PROB. THE CAPT DECIDED TO PULL OUT THE IRREGULAR PROC FOR HYD SYS 'A' QUANTITY OR PRESSURE LOSS. WE REVIEWED THE PROC; AT WHICH TIME THE CAPT CONTACTED MAINT AND DISPATCH OF A POSSIBLE PROB WITH THE LOSS OF QUANTITY. I FLEW THE ACFT AND WORKED THE RADIOS WITH ATC. DURING THE CAPT'S CONVERSATION WITH MAINT; I NOTICE A FURTHER DROP IN QUANTITY TO ABOUT 1/4 FULL ON SYS 'A' AND NOTIFIED THE CAPT; AT WHICH TIME SYS 'A' ENG DRIVEN HYD PUMP LOW PRESSURE LIGHT ILLUMINATED. WE THEN FOLLOWED THE IRREGULAR PROC FOR HYD SYS 'A' QUANTITY OR PRESSURE LOSS. SYS OPERATED NORMALLY UNTIL DSNDING FROM 10000 FT TO 8000 FT; AND TURNING FROM 315 DEG HDG TO A 290 DEG HDG; WHEN WE LOST THE LAST OF THE HYD QUANTITY FROM SYS 'A' AND ALL SYS 'A' PRESSURE. THE CAPT THEN DECLARED AN EMER WITH ATC REQUESTING AN EXTENDED DOWNWIND AND ALSO NOTIFIED THE FLT ATTENDANTS AND THE PAX OF THE SITUATION; WHILE I FLEW THE ACFT AND WORKED WITH ATC. I TURNED TO HDG 270 DEGS AND REDUCED SPD TO 210 KTS BEFORE THE CAPT STATED HE WAS GOING TO MAKE THE LNDG AND WANTED ME TO GO BACK OVER AND COMPLETE THE IRREGULAR PROC FOR HYD SYS 'A' QUANTITY OR PRESSURE LOSS. WE THEN XFERRED THE CTL OF THE ACFT; AT WHICH TIME I REVIEWED AND COMPLETED THE PROC FOR EXTENDING THE GEAR MANUALLY AND PLACED SYS 'A' ELECTRIC HYD PUMP SWITCH TO THE 'ON' POS AND ADVISED THE CAPT HE HAD SYS 'A' PRESSURE BUT HE MAY OR MAY NOT HAVE NOSEWHEEL STEERING UPON LNDG. THE CAPT LANDED AND ROLLED OUT TO TXWY; WHERE I ADVISED HIM THAT WE LOST SYS 'A' PRESSURE TOTALLY. THE CAPT SET THE BRAKES AND WE ADVISED GND CTL AND EMER EQUIP OF THE LOSS OF PRESSURE AND ADVISED THEM WE NEEDED A TOW TO THE GATE. THE GND CTLR STATED WE HAD SMOKE COMING FROM THE L SIDE OF THE ACFT AND THE EMER EQUIP WAS COMING TO INVESTIGATE BUT FOUND NOTHING. WE PROCEEDED TO START THE APU AND SHUT DOWN BOTH ENGS AND COMPLETED OUR CHKLISTS. WE WERE TOWED TO OUR GATE AND WERE DEBRIEFED BY THE MAINT PERSONNEL AT GATE. NO PAX OR FLT CREW WERE INJURED.