I have taken a look at the rear master cylinder when the pedal is pressed. You can see that the master cylinder moves by a heck of a lot.
It appears that the bracket that holds the master cylinder is not up to the job. I intend to make a new one from decent ally plate and add a method of fixing a small heat shield for the exhaust pipes.
I think that with the master cylinder being held firmly and correctly with no flex when the brake pedal is pressed and a heat shield could do the trick.
And yes, my brake works ok but I reckon it could be better.
DC