What can cause brake failure in Audi A6 99?
The brakes are separated into two separate sections, as a safety feature in case there is a failure in one section, the other section will still function. This is why only two of the four wheels are braking. The problem does sound like a master cylinder. The two separate braking sections of your car starts with the master cylinder, and the master cylinder then feeds the ABS system. The master cylinder can be rebuilt, but normally you get an already rebuilt cylinder at the car parts store & install it. They will probably want the old take-out cylinder back as a core.
