Lately my brakes have not been working. I'll be driving at say 40-60mph and then hit the brakes to slow down and they don't respond - the pedal just gets rock-hard and nothing happens.
Can someone tell me what the issue is?
Thanks.

Car - 2001 A4 72,000 miles, 1.8T.

Possibly a few different things - failing master cylinder, air in the system, glazed rotors/pads. There's a few different possibilities to investigate.

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99 2.8QM

"I've always kept an open, honest relationship with my daughter. Maybe that way she won't fall for the same sh*t her mother fell for." Robert Schimmel

The pedal gets rock hard? Usually when the master cylinder is going, air in the system, the pedal gets really mushy and has extra travel.

If the pedal is hard, doesn't move very far, and doesn't do much, it could be the brake booster.

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1998.5, Quattro, V6, 5Spd, Sport, STOCK

Yes, the pedal gets rock hard and the car cannot stop, obviously.

A hard brake pedal could be due to a clogged up reservoir. Have you tried sucking out the reservoir and flushing the brakes with new fluid?

Another possibility is an ABS issue. Do you have an ABS light?

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01 A4 1.8T Quattro Sport
Eibach, Hotchkis, Upsolute, Borla

No there is no ABS light on.
I have not done a flush yet.

I was told that my right front CV boot joint is torn and needs replacement.
Could this be what is affecting my brakes?

Possibly, if there is an ABS issue. The tone ring is on the outer CV. If it's cracked or full of grease, it could be an issue. Do you have an ABS light turn on with key on/engine off? How about if you test the instrument cluster with the knobs?

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01 A4 1.8T Quattro Sport
Eibach, Hotchkis, Upsolute, Borla

No, I do not have an ABS light at all.
What could it be?

Try a flush first. Brake fluid is cheap, it's a good way to see what's going on.

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01 A4 1.8T Quattro Sport
Eibach, Hotchkis, Upsolute, Borla