What was found
CREW OXYGEN SYSTEM-LOW. RECORD PRESSURE PRIOR TO SERVICING. CREW O2 LEAKING MORE THAN 20 PSI PER HOUR. USING LEAK CK, CHECK PRESS REG XMITTER, DISTRIBUTION MANIFOLD, AND LINES AT O2 BOTTLE FITTINGS. REMOVED CAPT O2 ACCESS PANEL FOR O2 BOXES AND CHECK LINES FOR LEAKS, NO LEAKS NOTED. LEAK CHECK CREW O2 BOXES, NO LEAKS NOTED. STILL LEAKING, NEEDS FURTHER TROUBLESHOOTING. CREW REPORTS O2 LOW. O2 LEAVING SFO WAS 990 PSI, AT ARRIVAL ORD 900 PSI. REPLACED CAPTS STOW BOX AND ALL 4 CREW MASKS, NO LEAKS NOTED 40 MINUTE LEAK TEST.