Long brake pedal

[HPsauce]

At least that's what my ex-garage-owner mate who just had a look says I should describe the problem as.

The brakes on my 1998 S8 feel pretty soft and not wonderfully effective; you have to push hard and a long way to get a rapid stop. No warnings.

My tame "expert" had a quick look and said, yes not working as they should, need attention sooner rather than later.
He did confirm that the rear pads were new (done late last year) and fronts had loads left. No signs of any leaks and the reservoir is full.

His opinion is that it was probably an internal leak in the master cylinder and that replacing it as a unit was the most likely solution.

Does anyone have a view on whether that is likely and if so how big a job is it and how much for parts and labour at an independent Audi specialist?

Thanks in advance.