Conan.
By George, I think you've got it!
Your explanation makes perfect sense as plainly what I was doing with the hammer (hitting it), would not affect the way that the piston travels.
I'm just surprised at Audi. I'd expected these pistons made out of Stainless.
On second thoughts, how come the 'pedal' brake don't stick on then?
Is the handbrake not the same piston?