A further note on the subject. Many years ago I had the pleasure of visiting the pressure chamber at RAF North Luffenham or maybe it was at Wittering? We ascended to around 35.000' wearing pressure breathing Oxygen equipment. When the masks were removed and we were asked to count backwards from 100, deducting three each time I think I got as far as 97, 94, 91, 80.......er....87....er....83 and so on. So the story of the RAAF Canberra is quite amazing.
Further to the SOP from the QRH in event of an auto fail, selecting MAN and driving the valve closed. There is the proviso that the valve should be moved until control over the rate of change is established. It is not a requirement to drive it fully closed and subject the cabin to a descent rate of 10.000' a minute. Goodness, that would have hurt!
As a final thought, if you do find yourself at 10.000' or MSA and consider the 'Emergency Descent' complete, just take the further descent to your landing field nice and easy. The pressure change at the lower altitudes is more marked at these lower altitudes so a descent at 3000fpm will be more of a strain on your body/tubes at this altitude than the 6000fpm you acheived at the upper altitude. Not so well put but I hope you get the point.