[vjck_uk]

I would'nt advise fitting a new master cylinder without bleeding the entire system properly.
Your brake fluid needs changing every couple of years as it absorbs water (being hydroscopic) and will corrode internally, calipers,pipes and master cylinder.
Plus the water content drastically reduces the boiling point of the fluid which can lead to entire brake loss after a sustained heavy braking session.
Just take a look at the old fluid when you bleed it.
It looks like someone has washed some potatoes in it.
Vaughan