Brakes go to the floor

I have a 1992 Corvette 5.7 . The brakes have started to feel kinda spongy
where sometimes I need to pump them slightly to get them to feel strong and
firm. After driving for 30 minutes the other day the brakes went all the way to
the floor and had no real stopping power. I had to drive home at 20 mph just
so I would be able to stop. I tried the car in my driveway yesterday just to see
if the breaks were still to the floor and they felt high and strong. I don't want
to chance it by driving it again but not sure what I should replace. The break
fluid is still full in the reservoir and there are no visible leaks at any of the
wheels and master cylinder. I am guessing it is either the master cylinder that
is failing or the power booster. Should I replace the master cylinder first or
take the car in and let a mechanic diagnose the problem? I am comfortable
replacing the master cylinder but not the power booster.