Brakes gone soft?? No brakes?? 986 Boxster

[Spinnaker]

Well, I suppose the first thing to do is to check all four corners for any signs of leaking.
I've never had to test a master cylinder before, they either worked or didn't. I would think that if you disconnected the brake lines from the master cylinder and put in some screwed in plug fittings, and then applied pressure to the brake pedal, it should have a solid pedal pressure since the brake fluid can't go anywhere. If it goes to the floor, you have your answer. Not sure how you would get the plugs in without introducing air in the system, but maybe you could screw them in partially and then bleed the air out like you would with a caliper. You would have to make some sort of accommodation to catch all the fluid that would be spilling all over. You don't want to get it all over the car and ruin the paint.
Someone with more recent experience may want to chime in if my process is flawed.

Edit: I suppose you could remove it from the car and test it on the bench. That way you won't get brake fluid all over the place. If it is bad, then you already have half the job done.