Brake line or master cyclinder?

Three days ago my ABS light came on. When I got home, I turned the car off and back on. The light stayed off. Today, driving to the store, I found myself in a scary situation where the brake pedal ended up going mushy on me and went all the way down. My brakes were there just barely, so I drove slow. Checked the fluid levels at the store, it was clear I was leaking brake fluid somewhere. Bought some more. Filled up and got home.
The ABS light comes on now when I do drive it. There are no puddles or signs of leakage by the wheels. There is a small stain under the master cyclinder. And where I parked there is wetness under that general area where I parked. My gut is saying it is the master cyclinder...that is an easy fix I can do myself. But if I'm wrong, then I'm wasting money and I don't have the money to fix the lines for a couple of weeks.
A second opinion would help tremendously. Thanks.