Get another opinion. You obviously have doubts, about the garage and or the mechanic.
To be honest i'd be asking very awkward questions as to why it wasn't picked up before that the brakes were binding or how the master cylinder is now all of a sudden at fault while there were no issues before the pipes were changed.
That said if it is brake master cylinder there is better time to buy one from our sponsors
Click here for 50% off brake master cylinder