Originally Posted by spindle
No, it's not a nylon valve. It's an actual metal bolt.
If the wings (handle) are they only thing turning then you might try spraying the heck out of it with a penetrating oil and after letting it soak for a good while,cut/pull the handle off and then use pliers to turn the base to see if it will open. Pliers to make that work will be a hassle but replacing the whole thing can be a BIGGER hassle.
If that won't work then replacing "the whole thing " is the next step. I would be thinking cutting an access would be the best way to do that.