how do you know its the master cylinder - is it leaking ?
usually they leak because the bores get worn (or the rubber cups 'perish')
master cylinder kits are easy enough to do -a set of circlip pliers and some brake fluid as lubrication and an evening fiddling to get it apart and cleaned and back together
however not sure what state (of OZ- not mind!) you are in but a quick trawl of the web should give you some clues as to people who can do exchange ones - to save the hassle - wouldnt think the Xy is that rare.
also if its leaking because of worn bores then best you get it checked as they usually leak soon after fixing if the bores are still stuffed....they can be resleeved with stainless - there is a multitude of people who can do this:
http://www.martinsclutch.com.au/brake/hydraulics.htm
hoppers stoppers
sunbury brake and clutch
etc etc
an xy on drums must be scary - i have an XP with drums and its a way lighter car and doesnt inspire me with confidence in the braking dept - maybe 40 years ago it was cutting edge - but in todays modern traffic..
pretty sure you could fit a set of front hubs/uprights/discs from any thing from XW to XD/E (maybe XF)+ booster and step into the 80's for braking performance .
and fairly cheaply .
or see hoppers website and go crazy with big discs
brett
melborne