on the S2/2a there is at least two different master cylinders for the braking system so first you need to know which one you have (i think mine is a CB from memory), i resealed mine without any problems, it cost me less than a fiver
i did buy a cheap replacement for the clutch master cylinder as the original had rotted but a clutch isn't as important as brakes (i've driven my 2a to get home with a busted cross shaft pin)
not all cylinders are scrap just because the seals have failed
it's always worth checking them out as a genuine replacement brake m/c for mine is somewhere around 100quid i think