I would imagine the easiest way to test a master cylinder would be to blank the outlets off.
This could be done with the assistance of 3 suitable sized bleed nipples ( Don't tighten them too much as you will risk damaging the sealing faces in the mc )
You could also make up dummy brake pipes, very short & either solder the ends or crimp them a couple of times in the vice.
Provided that you have not trapped a massive amount of air in the master cylinder, if all is well with the master cylinder, the pedal should be rock hard when pressed.
As someone said, it is not uncommon for the master cylinders seals to fail after a long period in storage, I have personaly come accross this situation a number of times
