Kevrumbo:
I had an unfixable freeflow on single tank at 6m deep in Cozumel this past August, with my dive buddy nowhere in sight: had to feather the tank valve to complete a relatively nominal MDL ascent.
Lesson learned Lynne --that is usable gas and you should try to conserve it by shutting it down initially if you can (i.e. you can even attempt to "feather & sip" any remaining breathing gas from the tank directly from the valve itself as a last resort measure). . .
I just had (this Sun) that same thing happen. I was solo diving at a spring, testing out my newly rebuilt Octo, when, at around 50 ft, I noticed it was leaking. I went over to a shallow platform, and knelt down to play with it.
Hummm.. leaking.. here, lets try breathing off of it..... nice, but boy the purge sure seems really touchy... still leaking... here, let me push the purge.... Wow.. look at the gas come out of this thing... Lets stop it..... wow, the thumb over the mouthpiece does not work.
I tried everything I knew to stop it... and ran out of air... Ok, in only 8 ft of water, and I just surfaced, but it went thru air really fast.. at 100 ft, I would never have made it to the surface.
Normally, I would have turned the tank off and retried it, but I don't think it would have made any difference, in that the valve is stuck slightly open. I will look at it this week, but bet there is something stuck in the seat.
In all these years, this was my first time to be the one out of air. I went through 1600 psi so fast... it surprized me.
Interestingly, I never pulled the pony reg and used it, but it was not by thought. I don't remember even thinking about using it. I was first playing with the Reg and then doing accent.