C5 S6 Intermittent Loss of Brakes
 

I recently purchased a C5 S6 and was making the long trip home when I lost 80% of braking performance. I noted right before that the brakes didn't bite until I had pushed a bit further on the pedal. I went to brake again and the car veered all over, clearly not getting equal pressure to each caliper. Not fun on the highway in traffic. I pulled over, fluid was full, no leaks. I drove another three hours home without issue. My usual mechanic has looked it over but found no issues. Brake pedal seems firm/normal when waiting at a light, doesn't move much. This is going to my wife/kids daily ride and I have to be 100% confident in it which I'm not. Pads on the car are good. Could it be the master cylinder? ABS module? I don't get the intermittent issue. When I've had master's go out in the past, they just go and pedal goes to the floor or is ultra spongy. I'm thinking I'm just going to do the master regardless. Thoughts would be much appreciated!

I spoke with a local German speciality shop and he mentioned they had an all-road with an issue with the hydraulic pump that would act up after an hour or two of driving. In my case, I'd been driving for over an hour and there was quite a bit of braking in traffic.

I noted a front caliper was replaced a year ago so we went ahead and bled the brakes and some air did bleed out. The pedal feels more firm than it did prior so hoping the issue is resolved.