Had a shop day yesterday working on the "Ol' Chevy".
Last time I drove it, went to a great car show, and lost nearly all brake pedal by the time I got home, not a good feeling. Checked the master cylinder and it was nearly empty, turns out my calipers were leaking. After much research, the Corvette forum is very good, this is a common problem.
So, it turns out the OEM calipers made by Delco, tend to have the "lip seals" leak when the car is not driven much. The calipers themselves are good and I've always been happy with the brakes on this car, but Chevy stopped using this type of seal in '82.
There is a company in TX that remanufactures these calipers, they install a SS sleeve and use an "o-ring" seal rather than the lip seal, so all should be golden! Not cheap but what is these days?