B767 Undifferentiated or Other Model
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 202; Flight Crew Total 12720; Flight Crew Type 808
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
General Maintenance Action
Aircraft; Procedure
During cockpit preflight checks; oxygen pressure read 1960 psi; but rapidly decreased to 70 psi when pushing and holding the oxygen mask emergency/test knob. After releasing the knob; pressure quickly returned to 1960 psi. Test returned same result on FO (First Officer) side.  Submitted ELB report and requested maintenance.  When maintenance reported to the flight deck and I demonstrated a failed test; they said that it was ok because the pressure was returning to normal after the test. I explained our test parameters and requested that they check that the oxygen cylinder valve was fully open. Maintenance found the cylinder valve only partially opened. Once fully opened the valve and the system then checked normallyWith the valve in the partially open position insufficient oxygen was available for breathing through the mask.  The aircraft was released from maintenance in this condition; where mechanics preformed ETOPS checks and issued a [Maintenance Release]. There were at least 3 opportunities to discover this problem before the airplane flew; and an additional opportunity before I discovered the problem.
B767 Captain reported discovering a crew oxygen issue during preflight that had been missed by multiple parties earlier.