A B747-400 WAS DISPATCHED AND RETURNED TO THE FIELD WITH THE CREW OXYGEN SUPPLY SYS LEAKING BEYOND DISPATCH LIMITS.

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ACFT XYZ INCURRED A RETURN TO THE FIELD AFTER OBSERVING CREW OXYGEN SYS PRESSURE LOST 120 PSI IN 2 HRS. MAINT FOUND A DEFECTIVE MASK QUICK DISCONNECT VALVE FITTING AT THE FIRST OBSERVER'S STATION. THERE WAS A CHAIN OF EVENTS THAT LED UP TO THIS PROB. THE OXYGEN SYS HAD BEEN PREVIOUSLY WRITTEN UP FOR A CHRONIC OXYGEN SYS LEAK. MECHS PROCEEDED TO CHK THE ENTIRE OXYGEN SYS FOR LEAKS. AT SHIFT TURNOVER; THEY UPDATED THE WRITE-UP STATING THE LEAK WAS ISOLATED DOWN TO EITHER THE THERMAL COMPENSATOR OR REGULATOR. MYSELF AND 2 OTHER MECHS FURTHER TROUBLESHOT THIS AREA AND FOUND THE TRANSDUCER TO BE DEFECTIVE GIVING FLUCTUATING (AND LOW) PSI INDICATION AT THE GAUGE. AFTER INSTALLING THE NEW TRANSDUCER; THE 2 OTHER MECHS FAILED TO RECONNECT THE SUPPLY LINE. THUS; WHILE PERFORMING LEAK CHK; OXYGEN WAS NOT FLOWING TO MASKS (WHERE THE LEAK EVENTUALLY TURNED OUT TO BE) AND OXYGEN SYS LEAK HAD APPEARED TO BE REPAIRED. THE FOLLOWING DAY AT THE GATE; THE FLC NOTICED NO OXYGEN FLOW AT THE MASKS DURING PREFLT CHKS. GATE MECHS COULD NOT IDENT THE PROB AND REQUESTED MY ASSISTANCE. UPON ARR; I INSPECTED THE AREA WHERE WE HAD PREVIOUSLY WORKED; AND IMMEDIATELY IDENTED THE PROB -- THE OXYGEN SUPPLY LINE WAS STILL DISCONNECTED AND CAPPED OFF; THUS NO FLOW TO THE MASKS. AT THIS TIME; OXYGEN SYS PRESSURE STILL READ NORMAL. THE GATE MECH RECONNECTED THE SUPPLY LINE. FLC CONFIRMED OXYGEN FLOW AT THE MASKS AND FLT DEPARTED 51 MINS LATE. 2 HRS INTO FLT; CREW OBSERVED 120 PSI LOSS IN PRESSURE AND RETURNED. MECHS FOUND SOURCE OF LEAK AT THE FIRST OBSERVER'S MASK QUICK DISCONNECT VALVE FITTING. REPLACED ASSEMBLY; AND LEAK CHK OK. CONTRIBUTING FACTORS TO THE CHAIN OF EVENTS THAT OCCURRED: THE MECHS FAILED TO CHK EACH OTHER'S WORK AFTER PERFORMING MAINT. ON THE SYS BOTH MECH HAD FOCUSED ON THE PRESSURE GAUGE AND DIDN'T NOTICE THE SUPPLY LINE WAS STILL CAPPED OFF AT THE 'T' FITTING. POOR LIGHTING AT THIS LOCATION WAS ANOTHER CONTRIBUTING FACTOR.