A B737-200 RETURNED TO THE FIELD DUE TO AN APPARENT LOSS OF 'A' SYS HYD QUANTITY.

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NORMAL DEP OUT OF COS UNTIL I RAISED THE LNDG GEAR. THE 'A' SYS LOW QUANTITY LIGHT ILLUMINATED (HYD) AND THE 'A' SYS HYD QUANTITY GAUGE INDICATED 1.8 GALS OF FLUID REMAINING. WE PROCEEDED TO WATCH THE GAUGE; IT CONTINUED SHOWING A DECREASE IN QUANTITY. WE ELECTED AND AGREED TO RETURN TO FIELD (COS); LEVELED AT 9000 FT; TURNED DOWNWIND FOR RWY 17L. I INFORMED THE CABIN CREW THAT WE WERE RETURNING TO COS AS A PRECAUTIONARY MEASURE WITH A POSSIBLE HYD LEAK AND WOULD BE ON THE GND IN APPROX 4 MINS. I THEN COMMUNICATED WITH COMPANY AND INFORMED THEM THAT WE WERE RETURNING TO COS WITHIN A FEW MINS. I ALSO INFORMED THE PAX OF OUR SIT. WE COMPLETED THE LDNG CHKLIST APPROX 800 FT AGL. I THEN ASKED THE CAPT IF THERE WAS ANYTHING ELSE HE NEEDED ME TO ACCOMPLISH AND HE STATED THAT HE WOULD LIKE ME TO MONITOR THE REMAINDER OF THE APCH AND LNDG WITH HIM. I THEN MADE THE REQUIRED 500 FT AGL CALLOUT WE LANDED NORMALLY WITH NO FURTHER PROBS AND PROCEEDED TO THE GATE. MAINT CTL CONTACTED US AS WELL AS CONTRACT MAINT; THEY INSPECTED THE ACFT; FILLED THE HYD SYS; FOUND NO LEAKS.