I also believe its "pad knock off" but have been unable to find the source of the flex. You are right, no floating parts here. My perplexity here is that I have 2 other cars with the exact same brake setup and neither of them do this at all.

Definitely not exhibiting any characteristics of a failing master cylinder.

It's the run up to SEMA right now and I have not had a chance to work on the Cossie, too busy with a few builds for the show. Talk about a brake setup, I am putting a 15" front and REAR setup on a car right now! As soon as get back, I am going to dig deeper in to this one.

Thanks for your help!

And here is another picture.