They are replaceable, but they have become extremely hard to find. We drill them out completely and replace the entire valve with one with a manual pressure relief valve like you would find on a ball lock style keg.
Here is what we use:
You have to drill a 3/4" hole with a hole saw. Frankly, it is a pain to do and we have a special jig for it, so it might be less trouble to replace the lid. Wish I had a better answer. Maybe someone else will know where to get the part.