A B737'S HYD FLUID ENTERED THE ACFT'S UNPRESSURED AIR CONDITIONING SYSTEM DUE TO THERMAL EXPANSION ON A HOT DAY. MAINT DRAINED FLUID FROM HYD RESERVOIR.

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AS SOON AS I ENTERED THE FLT DECK I DETECTED AN ODOR SIMILAR TO A DIRTY SOCK SMELL. THE AIR CONDITIONING PACK WAS SHUT OFF; AND THE APU WAS SHUT DOWN. I THEN LEFT THE FLT DECK AND REQUESTED THAT THE FLT ATTENDANTS LEAVE THE JET. MAINT WAS NOTIFIED OF THE ISSUE AND THE PROB WAS RECORDED IN THE ACFT MAINT LOG. A MECH SHOWED UP AND CLAIMED HE WAS VERY FAMILIAR WITH OUR PROB AND WHAT WAS CAUSING IT. ULTIMATELY IT WAS DETERMINED THAT ONE OR BOTH HYD RESERVOIRS WERE OVER-SVCED AND AS A RESULT OF THIS HYD FLUID WAS ESCAPING INTO THE PNEUMATIC SYS; AND ULTIMATELY INTO THE AIR CONDITIONING PACKS. APPARENTLY THE HYD FLUID WAS IN THE EARLIEST STAGES OF GETTING INTO THE AIR CONDITIONING PACKS. THE FIX FOR THE PROB WAS THAT NEARLY 3 GALS OF HYD FLUID WAS DRAINED FROM THE RESERVOIRS BEFORE THE AIRPLANE WAS PLACED BACK IN SVC. THE MECH EXPLAINED TO ME THAT THE RESERVOIRS WERE INITIALLY SVCED PROPERLY; BUT DUE TO 90+ DEG TEMPS OUTSIDE OVER A PERIOD OF SEVERAL HRS THE FLUID EXPANDED UNTIL ULTIMATELY SEEPING OUT OF THE PNEUMATIC HEAD PRESSURE OUTPUT IN THE TOP OF THE RESERVOIRS AND FINALLY INTO THE AIR CONDITIONING PACKS. MAINT NEEDS TO BE SURE THAT HYD RESERVOIRS ARE NOT OVER-SVCED; OR SVCED TO A POS IN WHICH EXPANSION COULD LATER RESULT IN AN OVER-SVCED CONDITION.