I have an 2001 a6 quattro 2.8. One day I got in the car and the brake pedal went to the floor. Ipumped it a few times and it got stiffer and then over the next 10 miles it got stiffer and stiffer and then the front right caliper stuck. I pulled over, took off the wheel and let out pressure and used the e brake to drive home. The caliper was fried. So I replaced the caliper, ended up replacing rotors, and pads while I was in there. I had all calipers off the car so most fluid was drained from them. Now I put it all back together and I dont have any pedal. Goes right to the floor. Tried bleeding and I am not getting any fluid to any of the calipers. The pedal will not get any stiffer. I have a vacum to do a one man bleed job too which I also tried. I hooked it up to the line to see if something was clogged. The one line I got about 2-3oz of fluid but the reserviour didnt really go down much. Everything seems tight and no leaks anywhere as the vacume will hold pressure when connected to each line. Is this a problem with the master cylinder? No pedal pressure at all....

 

master cylinder with edl most likely the culprit, check the left side rear caliper as well to see if it is locking up, these system tend to run an X pattern. I stepper on my brakes at 125mph and had the left rear and right front lock. Very interesting situation. Anyways the culprit was a failed master.

 

I bleed my brakes using a clear plastic hose going into a clear jar with some old brake fluid at the bottom. The hose must go to the bottom of the jar. The jar must be located above the brake caliper. The hose must fit tightly on the end of the bleeder screw.
You may have air trapped inside the master cylinder or a defective master cylinder. On some cars the master cylinder has to be bleed before installation. I don't know if this is true for an Audi.
Try putting the jar and hose on one of the calipers, then press the brake pedal and then slowly release it. Do this several times on each caliper, until there are no more air bubbles in the hose.