typical symptoms of failing brake MC - is it the MC or the booster?

[David Pertuz]

a quickie -

Every ocne in a while when I am driving and hit the brakes, my pedal will
sink an inch or two with no braking, and then  the brakes will pick up
again, albeit weakly. This has happened maybe 3-4 times in the last few
months. I understand this to be the typical symptom of a failing brake
master cylinder, but does this mean a failing booster, or the MC itself? It
would seem to me ot point to a booster (internal vacuum leak, maybe) but
I'm not sure so I'd like to ask so that I can deal with the correct part.

thanks,
david