B727 FLT CREW HAS LOSS OF 'A' HYD SYS ON DEP FROM MMUN.

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ON CLBOUT; 'A' SYS HYDS SHOWED A SLOW LEAK. 'A' SYS PWRS ITEMS SUCH AS FLAPS; LNDG GEAR AND NOSEWHEEL STEERING; ONLY 25 MINS AFTER TKOF OVER THE GULF OF MEXICO AT NIGHT IN TSTM CONDITIONS; I ELECTED TO RETURN TO POINT OF DEP IN CANCUN; MEXICO. ANOTHER FAILURE ('B' SYS) WOULD RENDER THE ACFT UNSAFE BECAUSE CTLS WOULD OPERATE MANUALLY AND WOULD BE HARD TO CTL AT HIGH SPD. AT THE COMMENCEMENT OF THE 'A' HYD SYS CHKLIST AND DECLARATION OF AN EMER; I ELECTED TO RETURN TO CANCUN. OUR WT WAS 164000 LBS. WHEN THE CHKLIST ADVISED TO DUMP FUEL TO GE TO 161000 LBS (MAX LNDG WT); I ELECTED TO MOVE ALONG ASSUMING WE WOULD BURN THE 34000 LBS OF DSCNT AND APCH. THE FE WAS BUSY ENOUGH RUNNING CHKLISTS TO WORRY ABOUT DUMPING 1000 LBS OF FUEL. PROVISIONS ARE MADE PER THE AIRLINE MANUAL FOR AN OVERWT LNDG; IN CASE OF EMER. CREW AND PAX WERE BRIEFED AND LNDG OCCURRED AT 300 FPM. 360 FPM OR GREATER REQUIRES A HARD LNDG CHK PER THE MANUAL. ONCE STOPPED; GND CREW ATTEMPTED TO TOW AIRPLANE BUT #1 AND #2 BRAKES DID NOT RELEASE AND PAX HAD TO BE BUSSED TO TERMINAL.