I had this issue as well. It turned out to be the proportional valve for the brake lines. It is to the left of where you put the brake fluid in, and is usually mounted against the wall that turns into the dash. Just look for a leaking valve the is " Y " shaped.
Also looking under the car to find the leak will make people suspect the master cylinder. Not saying that's wrong, just what I thought at first as well.
Oh and my car is a Kia Spectra 2002.