A B727-200 ON INITIAL CLB AT 12000 FT DECLARED AN EMER AND DIVERTED DUE TO LOSS OF 'A' SYS HYDS PRESSURE AND QUANTITY; CAUSE UNKNOWN.

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AT 12000 FT; 10 MI OUT OF DIA; ENRTE TO PUERTO VALLARTA; WE LOST HYD LIQUID IN THE SYS 'A' (MAIN ONE). I DECIDED TO RETURN TO LAND BECAUSE THE LIMITATIONS OF AIRSPD AND ALT WITH A YAW DAMPER INOP AND BECAUSE THE BAD WX ALONG THE RTE. AT THAT POINT THE GROSS WT OF THE ACFT WAS 80000 KILOGRAMS; 10000 KILOGRAMS OVER THE MAX LNDG WT. A TSTM WAS 10 MI E OF THE ARPT WITH LIGHTNING AND THE WIND BEGAN TO INCREASE AND BE VERY VARIABLE AT THE ARPT; SO I DECIDED TO LAND WITH THE OVERWT. (THIS PROC IS IN THE OPS MANUAL OF THE COMPANY.) THE VREF FOR THAT WT WAS 158 KTS AND THE APCH WAS MADE AT VREF +15 KTS AS THE PROC SAYS. THE TOUCHDOWN WAS AT 165 KTS APPROX AND THE BRAKING MODERATE. I TURN OUT OF THE RWY 35R IN THE LAST HIGH-SPD TXWY AND BEGAN TO TAXI VIA TXWY G TO THE GATE WHEN WE LOSS ALL THE HYD AND THE NOSE STEERING FAILS; SO I STOPPED THE ACFT AND REQUEST A TOW CAR TO GO TO THE GATE. WHEN THE GND CREW ARRIVED; THEY INFORMED ME THAT THE TEMP FUSES OF THE TIRES WERE POPPED AND THE TIRES WERE LOSING AIR SO WE REQUESTED BUSES TO DEPLANE THE PAX (173 PAX).