AN A320 FLT CREW EXPERIENCED A 'HYD SYS QTY LOW' ECAM. AFTER CONSULTING WITH DISPATCH AND MAINT; THEY DECLARED AN EMERGENCY AND DIVERTED TO THE NEAREST SUITABLE AIRPORT.

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IN PREFLT; I NOTICED THAT THE GREEN HYD SYS APPEARED TO BE OVER-SVCED. I NOTIFIED MAINT; AND MADE A LOGBOOK ENTRY FOR THIS ITEM. THE MECH DRAINED SOME OF THE HYD FLUID AND CLRED THIS LOGBOOK ITEM. AFTER COMING UP TO THE COCKPIT; HE RETURNED TO THE L WHEELWELL AREA IN ORDER TO PACK UP HIS EQUIP. THE FO AND I NOTICED THAT THE HYD FLUID LEVEL IN THE GREEN SYS APPEARED TO BE EVEN HIGHER THAN BEFORE; AND THAT THIS SYS ALSO NOW HAD A 'LO AIR PRESS' FLAG AS WELL. I EXPECTED THE LOW AIR PRESSURE SINCE THE MECH HAD SVCED THE SYS; BUT I WAS CONCERNED ABOUT THE FLUID LEVEL. I WENT OUTSIDE AND ASKED THE MECH IF THIS WAS NORMAL. HE ASSURED ME THAT AS THE SYS'S AIR PRESSURE BUILT UP; THE FLUID LEVEL INDICATION WOULD RETURN TO NORMAL RANGE. WE CLRED THE ASSOCIATED ECAM MESSAGE FOR THE LOW AIR PRESSURE; AND CONTINUED TO PREPARE FOR DEP. WE PUSHED BACK; AND STARTED THE #1 ENG. AT THE END OF THIS ENG START WE HAD A MOMENTARY ECAM MESSAGE; BUT I DIDN'T SEE WHAT IT WAS BEFORE THIS MESSAGE CLRED ITSELF. WE STARTED TO TAXI OUT TO THE RWY; AND WHILE DOING SO; I CHKED THE HYD ECAM PAGE. THIS PAGE STILL SHOWED THAT THE GREEN HYD SYS HAD LOW AIR PRESSURE. I CONTACTED MAINT AND HE INFORMED US THAT WE WOULD NEED TO RETURN TO THE GATE. MY FO COORDINATED OUR RETURN TO THE GATE WITH ATC AND THE RAMP. WE HAD TO WAIT FOR A GATE; AND WE INQUIRED IF MAINT COULD JUST MEET US ON THE RAMP WHILE WE WERE WAITING. THEY WERE UNABLE TO DO SO. WHEN WE GOT TO THE GATE A MECH FOUND THAT THE PRESSURIZATION VALVE FOR THE GREEN HYD SYS WAS STUCK OPEN. HE CLOSED IT; CAME UP TO THE COCKPIT; AND VERIFIED THAT THE SYS WAS NOW PRESSURIZING. IT WAS A VERY QUICK FIX. UNFORTUNATELY; BEFORE WE COULD PUSH BACK FROM THE GATE; THE RAMP PERSONNEL WERE CLRED OFF THE RAMP DUE TO LIGHTNING. WE FINALLY PUSHED; AND STARTED THE ENG. WE HAD A MOMENTARY ECAM MESSAGE FOR THE SYS LOW AIR PRESSURE; BUT WE VERIFIED THAT THE SYS WAS NOW PROPERLY PRESSURIZED. WE DEPARTED UNEVENTFULLY. JUST BEFORE THE TOP OF DSCNT WE RECEIVED AN ECAM CAUTION WHICH STATED THAT THE GREEN HYD SYS NOW HAD A LOW QUANTITY. I DIRECTED THE FO TO PERFORM THE ECAM PROC WHILE I MONITORED THE ACFT AND THE RADIOS. WHEN HE FINISHED THE ECAM; WE REVIEWED THE FLT MANUAL; AND I BRIEFLY NOTIFIED THE PURSER OF OUR PROB. I THEN DIRECTED THE FO TO MONITOR ATC AND I HAD DISPATCH CONTACT ME WITH MAINT. WE BRIEFLY DISCUSSED THE SITUATION; AND I INFORMED DISPATCH THAT THE FO AND I THOUGHT THAT DIVERTING TO ZZZ WOULD BE THE SAFEST OPTION FOR OUR FLT. DISPATCH AND MAINT AGREED WITH US; AND THEY INFORMED ME THAT THEY WOULD COORDINATE WITH ZZZ FOR US. I THEN CONTACTED THE PURSER AND A FLT ATTENDANT IN THE REAR OF THE ACFT. I DIRECTED THEM TO PERFORM A 'CABIN ADVISORY' AND EXPLAINED TO THEM THE NATURE OF OUR PROB; THE TIME WE HAD UNTIL LNDG AT ZZZ; AND THAT I WOULD MAKE AN ANNOUNCEMENT TO THE PAX IN ABOUT 5 MINS. THE FO AND I PREPARED FOR OUR APCH AND LNDG; I BRIEFED THE PAX; AND WE LANDED UNEVENTFULLY. WE STOPPED STRAIGHT AHEAD ON THE RWY; AS WE HAD NOTIFIED ATC WE WOULD; AND THEN WE WAITED A VERY LONG TIME FOR A TUG TO COME TO OUR ACFT AND TOW US TO A GATE. SUPPLEMENTAL INFO FROM ACN 798886: FLT WAS UNEVENTFUL UNTIL G HYD QUANTITY ECAM AND THE QUANTITY INDICATOR WAS VERY LOW. WE DECLARED AN EMER AND DIVERTED.