AFTER EXPERIENCING THE LOSS OF THE 'A' HYDRAULIC SYSTEM QUANTITY; A B737-300 CREW DECLARED AN EMERGENCY AND LANDED AT THE DEST NEARBY.

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JUST BEFORE DSCNT INTO ZZZ THE FO NOTICED THE A HYD FLUID WAS DOWN TO 25 (FROM 90). NO WARNING LIGHTS WERE ON AT THE TIME. WE GOT THE BOOK OUT FOR THE A LOW QUANTITY/PRESSURE. SHORTLY AFTER THAT; WE GOT THE ENG A HYD LOW PRESSURE LIGHT ON. WE FOLLOWED THE BOOK PROC WHICH TURNS OFF BOTH A PUMPS IF THE FLUID IS BELOW 25. THE FLUID NOW WAS AT 21. WE SHUT OFF BOTH PUMPS AND TURNED ON THE STANDBY PUMP AND FOLLOWED THE REMAINDER OF THE FLT MANUAL PROC. I NOTIFIED ATC THAT WE WERE DECLARING AN EMER FOR LOSS OF A SYS. I CALLED THE PURSER AND TOLD HER OUR SITUATION AND PUT THEM ON A CABIN ADVISORY EXPECTING A NORMAL LNDG. I NOTIFIED THE PAX THAT WE HAD LOST 1 OF OUR 3 HYD SYS AND THAT WE EXPECTED A NORMAL LNDG IN ZZZ. I NOTIFIED DISPATCH AND SENT A STATUS RPT TO MAINT THAT WE HAD DECLARED AN EMER AND WERE COMING IN WITH A COMPLETE LOSS OF A SYS. ATC WAS EXCELLENT. I ASKED FOR A 20 MI FINAL AS WE HAD TO LOWER THE GEAR MANUALLY. I ASKED FOR THE LONGEST RWY. WE WERE CLRED DIRECT ZZZ FOR RWY XXL. I HAD THE FIRE TRUCKS STANDING BY. LNDG GEAR DEPLOYED PROPERLY WITH THE T-HANDLES; WE HAD ALTERNATE NOSEWHEEL STEERING AND IT WORKED PROPERLY. LNDG WAS NORMAL AND WE WERE ABLE TO TAXI NORMALLY. WE WENT TO THE B DEICE PAD AND MAINT BROUGHT A TUG. WE STARTED THE APU; SHUT DOWN THE ENGS AND WERE TUGGED TO THE GATE. APPARENTLY THE O RING FAILED AT THE ELECTRIC PUMP AND DUMPED THE FLUID OVERBOARD. THE FLT ATTENDANTS WERE EXCELLENT; ATC DID A SUPERB JOB AND MY COPLT (WHO IS NEW) WAS STUNNING. COULDN'T ASK FOR BETTER HELP ALL AROUND. ALL BACKUP SYS WORKED PROPERLY. REMARKABLE REALLY.