A certain UK company that I fly for changed their O2 test procedure to run it for 5 secs since I found the bottle had been turned 99.9% off and the standard test of "one pull" on the "Emerg 100%-test" lever wouldn't drain the line down enough to show a drop on the pressure gauge. It would refill slowly but when I put on the mask it sucked onto my face like a limpet. 5 sectors since maintenance.
Remember the KingAir off the coast of France? Reminds me of the old adage about Cessna fuel gauges. "The only thing it can reliably tell you is whether there is one fitted or not."