What was found
CREW OXYGEN PRESSURE LEAKING DURING FLIGHT FROM O2 MASK BEHIND C A ON ARRIVAL CREW NOTED; OXYGEN LEVEL AT 1620 PSI ON DEPARTURE. BY ARRIVAL CREW O2 DEPLETED TO 1460 PSI. CAPT NOTICED O2 MASK BEHIND (CAPT SEAT) LEAKED. PERFORMED GVI OF AFT (LH) STOWAGE BOX PER AMM TASK 35-11-51-200-801, DID NOT NOTICE ANY MAJOR DISCREPANCIES. WILL REQUIRE FURTHER INSPECTION FOR LEAKS. SERVICED CREW OXYGEN TO 1730 PSI IAW AMM 12-15-08-613-110. REMOVED AND REPLACED OBSERVER 2 LEFT SIDE COCKPIT CREW OXYGEN MASK IAW AMM 35-11-51-004-059 AND AMM 35-11-51-404-060. OPS CHECK GOOD LEAK CHECK GOOD.