I am looking at a 1972 Skylark that was stored for several years. The car is in good shape but has 2 issues. This post is a question about the brakes. The car has power front disc brakes. While driving the car, the brake warning light is coming on intermittently on the dash. The master cylinder is a little rusty on the outside, but doesn't look too bad. Inside, the fluid is full. The car seems to stop OK, but I thought I felt a little fade when driving it (maybe psychological?). Any thoughts on what could be causing this?

Is this meant to mean when using the brakes?

"While driving the car, the brake warning light is coming on intermittently on the dash."

If the ligh is coming on when not using the brakes it most likely is a bad sending unit or an out of adjustment switch on the emergency brake. Try pulling up on the emergency brake every time you drive for a while and see if that fixes the problem. If not try a new sending unit.

If it is when using the brakes the first thing to do is to bleed the brakes and put in new fluid. If that does not correct the problem I would replace the sending unit next.

If you have pedal fade and no fluid loss, then your problem is internal to the M/C. One or more of the rubber cups inside the M/C bore are either hard or nicked causing fluid to bypass. Replace it, its cheap insurance. Just make sure that you bleed the new one properly.