Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 125; Flight Crew Total 19000; Flight Crew Type 7103
Communication Breakdown; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
General Maintenance Action
Equipment / Tooling
We arrived at the airplane from layover; and preformed our preflight duties. When I checked the O2; the pressure dropped approx. 400 lbs. and stayed there - recovering after the test. I asked the F/O (First Officer) to test; and got the same result. We then tested at the same time; and the pressure dropped about 850 lbs. I called local ops for the mechanic; wrote it up in the maintenance (MX) log book; and sent an ACARS message to MX. When the contract mechanic showed up and I demonstrated the problem; he called MX control. They told him it was within limits. I requested to speak to the MX controller; who said to me that nothing was wrong since the pressure while testing; was still above allowable (approx. 950lbs). I explained that it was definitely NOT normal. That I've flown 737s for 10 years; and this has never happened before. I got the run around that it was a contract MX base and no parts; and who knows how long it would be before it could be corrected to my satisfaction. I indicated that the mechanic was here; and could inspect the system. I was particularly concerned that the valve was only partially open (Bernoulli's principle). MX control spoke again to the contract mechanic; who went below and inspected the system. He returned 5 minutes later - problem solved: The valve was less than one full turn open; and safety wired. He clipped the wire; turned the valve about 7 turns to full open; and reinstalled the safety wire. The system then tested normally.Whomever worked on the O2 system did not follow procedure - possible catastrophe. The pilots who brought the plane in (and all those who flew it after the O2 was improperly worked on); either did not preflight the O2 system; or did not investigate a non-normal indication - possible catastrophe. I felt pressured by the MX control that I was writing up a nonissue - With a new captain; or weak personality - possible catastrophe.
The shut-off valve on the Crew O2 Bottle was only open a small amount; therefore the O2 pressure would not recover in a timely manner.