2000 Other Chevrolet Models brakes mushy and brake light on

This is a 3500 service body I pulled up to a job everything was fine. 2 hrs pass and I go to leave. the brake pedel goes to the floor and the brake light goes on. I stop at the autoparts store and fill brake fluid ( about 1/2" low). No difference, no fluid leak. Is this something to do with the master cylinder. I am pretty good with repairs but I dont know where to start.