1996 Oldsmobile 88 no brakes

As the aughter exited the highway she had to press the brake pedal to the floor. Heard a strange noise and limped the car to a parking stucture, each time had to press the pedal to the floor to stop. Just as she was parking the brake warning light on the dash lit up and a warning bell sounded. I went the next day found the brake fluid gone but not leaks or wettness found. Refilled fluid resivour started the engine and then someone pumped the brakes. Thepedal stiffened a little as the fliud slowly left the resivour. A thumping soud froum the round disc containor behind the master cyclider was making a whoos soud each time the pedal was depressed. I have an idea of the problem but need to confirm.