Master cylinder back pot empty

I got a 1970 340 Duster. It has front disc brakes that are factory. went for a hour ride yesterday and when I got home I went to park it and the pedal went to the floor. Pulled the cap off the master cylinder and the front pot was full of fluid but the back one was empty. check under the car and there is no wet spots or any signs of fluid leaks,inside tires are dry as are all lines . I am open for any suggestions or help you mopar GURU's can send my way, thanks Steve.