Brake problem on 1997 Pontiac Firebird

I bought a 1997 Firebird recently, and everything was going fine until two days ago. All of a sudden, the brakes got really "squishy" or "spongy" and I have to press them down much further in order to stop. Tonight, the "Brake" light on the instrument cluster came on. It's not the "ABS Inop" light, but the red light like when you apply the parking brake. I haven't seen any brake fluid on the garage floor - any ideas?

brake shoes on the rear.just happen to me to, but was on a ford tempo.do not push on the peddle anymore because you will blow out the rubber boots and seals on wheel cylinders.parking break will adjust automaticly when new shoes are installed.anytime parking light comes on like that with no other conditions present its the rears.so what i would do, like i did on the tempo and what i did is to change front disk pads also.So with your luck it might be in the middle of the winter the fronts would need changing.your right their you might as well spend a weekend get-er done and over with. then it would be good for a year or so.i change all rotors and shoes on all three of my cars every two years weather they need it or not.i do my own work, since i was sixteen and am 62 know and still work on my own cars. i trust no one.parts are nothing to me compare with safety.
my wife and my children and grand babys go shopping in these cars my wife is handicap.be safe then sorry pads and shoes are cheap compared to life and limb and the lose of family.it also allows me to inspect under wears of each car.when momas happy everything body happy.she likes peddle travel up tight because of her handicap with her legs.

Do you think it could be a master cylinder problem rather than the rear drums? I had no indication of any brake problems prior to this happening.

brake light comming on indicates parking brake. which is connected to rear brake shoes. master cylinder is possible but cheapist way frist. I still bet rear shoes.