[IanS]

The actual bung was loose and could be slid completely out. I fixed it with jbweld, and pressure tested it afterwards with no problems. I don't think it was originally welded in there, I believe it was some type of epoxy/adhesive.

The best way to check a DV is to use a vacuum pump and see if the small nipple holds vacuum. It should. If you don't have a vacuum pump, take off the DV, push the valve open, put your finger on the small nipple, and the DV should stay open. This is a bit less reliable since it can be hard to get a good seal with your finger.