It occurred to me this afternoon that I did know what the valve on my mom's mask was for when I was trying to fix it. I read it in the directions on how to reseat it. We did pretty much what LinkC said to do, and it didn't work.
I'm a pretty good tinker, but the conclusion I came to is that it would be very difficult to fix, and given it's purpose, I didn't want to mess with it. Having that valve stick open is one thing, but you don't want it to stick closed. Definitely not the place for bailing wire, duct tape, and plumber's epoxy putty.
Of course, you've got a different model and I may have missed something. I am definitely not remembering clearly.
Even if it's not under warranty, it wouldn't surprise me too much if you could get it replaced for free. "I had one of those but the anti-asphyxiation valve went bad," doesn't sound like something FP's marketing dept. wants to read on the internet.