reaction to the answers:
There is a problem in the system in that airplane.
That is EXACTLY why you do the initial short test. You should see NO pressure drop.
A drop of 100psi during the test is acceptable according our FCOM, so there shouldn't be the problem.
The lowest our table -the company put one in our QRH- goes is 8000ft; but there 1500psi should give you 5:50 manhours of 100% oxygen on the 744F. I suppose the flightdeck system is similar to the pax -400.
the system on the Freighter is different than the Pax version. the freighter has more bottles for fire surpression. However i like to see these tables...
I am still shocked to see that we only have 30 minutes of oxygen in the 744-400