With my AUS (Google AMS800), the most likely point of failure is loss of pressure in the reservoir, and it is that that holds the cuff in the closed position, so it fails safe (i.e. open). The AMS800 is quite an old design now, and has been through a few refinements. There is a later one - the details are on the message board somewhere - do a search for "AUS". If you can't find the reference, I'll have a look for it when I am back later this evening.