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I was having a conversation with a buddy of mine and he was telling me about a "friend" of his that supposedly had regulator issues at depth, but his BCD would still inflate. He told me his friend said that he was able to make a swimming ascent by adding air into his BCD, then immediately putting his oral inflator into his mouth, pressing the deflate button, and breathing, but with some minor difficulties due to water. My friend told me that he "called his bull****", but I personally think that it may be *possible*, assuming you could crane your head around to get the hose in an upright position to breathe
It sounds like crap to me. You would need two simultaneous second stages that failed closed, which is less likely that winning the lottery.
While it is possible to move air from your BC to your lungs and back again if you want, it accomplishes nothing except giving you an opportunity for an astonishingly nasty lung infection.
If you don't have anything to breathe, share with your buddy. If you lost your air and your buddy, do an Out of Air Ascent
Just follow training. You'll live longer.
flots