Master cylinders often fail apparently for no reason, but coinciding with doing other brake work necessitating rebleeding the system.
The reason is that in normal use the plunger inside is only ever partly depressed, so a portion of the cylinder bore is never swept by the piston and seals. Dirt and corrosion build up on that portion, until the moment when the pedal is pushed vigorously, and the seal is then scraped over the rough portion of the bore. This can damage the seal and make a previously good MC seem spongy, despite repeated bleeding.

So if you are fitting a secondhand MC of unknown age/condition, I suppose the best thing would be to dismantle it, examine and polish the bore carefully, and then reassemble with new seals. In my own view however using old MCs is a waste of time. New ones are not expensive.

Whatever you do however, you can swap it on without having to rebleed the whole system. There have been several threads on the subject, and I have previously posted on how to replace a MC without any rebleeding.