Hi Mick. Without hearing (or seeing) the file in question, it's a little tough to nail down the exact problem. However, here's a few things to consider:
Brush size: Make sure the brush size is no larger than it needs to me. Smaller brushes will allow you to make more precise selections without removing any more of the material you want to keep.
Ring frequency. If the ringing happens to correspond to any of the frequencies of the material you need to keep, then the attenuation process will invariably remove some of the volume of the voice. That is also why having an appropriately small brush size is so critical.
It sounds like you're doing all the right things, so I'm not sure why so much of the voice is being removed. If you'd like me to take a whack at it, make a short version of the file and include a link where I can download it. After I hear and see it, I can offer more specific solutions to the problem.
Matt