Brakes went out

My girlfriend has a 92 Ford Taurus, out of no where he brakes completely went out. The brake pedle lost all pressure and brakes were barley applied when it reached the floor. She was a little low on brake fluid which i thought was the first problem and filled it up. However it did not fix the problem. Would bleeding the brakes fix it? Or might it be some more complicated problem?

Check the entire system for leaks. The master cylinder may be bad.

How would I check the ENTIRE system for leaks? I knw its a leak for sure now because now its empty.

Start with the lines going to the master cylinder and follow them to each wheel. Making sure lines aren't ruptured or seals in the calipers and/or wheel cylinders aren't leaking. Reccommend getting a repair manual and becoming familiar with all the components in the brake system. Master cylinders can leak from the rear seals causing brake fluid to drip down the pedal.

Where is the most common spot that leaving occurs? Caliper Seals?