C7 A6 (2012) Very Odd Brake Issue HELP!

[Flossabe]

I'm having this very odd brake issue that is somewhat concerning and I haven't seen anything described like it on here.
Car: 2012 C7 A6 3.0T Prestige

Usually happens while driving on the highway, 70-80 mph, but has happened on surface streets.
When applying the brake after cruising for a bit the pedal goes half way to the floor before building any resistance or applying any braking effect.
After pressure builds the car brakes like you would expect. In other words you have to pump the brakes once before any real braking happens.
This is very noticeable and consistent and +90% happens on the highway.

All pads are good, dealership and myself have blead the brakes.
After pumping brakes pedal feel is solid and brakes well, happens again after cruising along for a bit (I haven't timed how long).
This is obviously not safe.

Any suggestions?

[John_Kaufmann]

Master cylinder might have a damaged seal on the piston pushing the fluid. I'm using old school information here for older cars but it might still apply.

Sounds AF though!!!!

I'd put an image of it here but damn Tapatalk uploading keeps failing!

https://www.liveabout.com/symptoms-of-master-cylinder-failure-4154925

Sent from my SM-T290 using Tapatalk

[GIACuser]

Sounds like air in the line to me.
Bleeding these in order is important
1. Passenger Rear
2. Driver Rear
3. Passenger Front
4. Driver Front

[squirrels]

Do the brake lines on the C7 run past any hot exhaust components that have a heat shield protecting the brake lines from heat? It seems that this could resut in boiling fluid in that spot of the brake line if the heat shield were missing. I just don't think Audi would run brake lines that close to hot exhaust components.

Have you verified that you do not have a caliper dragging when this happens? A way to tell would be to immediately stop after this behavior is observed and check the temperature of all rotors, the fronts should be relatively close to each other, as should the rears. You would probably smell this, but then again I haven't smelled my dragging brakes in the past until the rotors were glowing. If a caliper gets hot enough, once again, you can boil the fluid. You would have to be reaching at least 230C with new fluid, but I have observed this before (not on an Audi, though).

Did the dealer bleed the ABS pump when they bled the brakes? I believe you have a Bosch unit but it is newer and I am not familiar with it yet, and I do not believe that VCDS has any ABS pump bleed function for this generation. I have never needed to do this on my C5 nor C6 but maybe C7 is different.

The only other thing I can think of is if you had some odd condition causing extreme (and intermittent?) lateral runout or deflection of one of the brake discs vs. the caliper, forcing the pistons on that caliper much further in than they would normally go, resulting in you needing to take up that extra gap when you hit the brakes... but this seems very unlikely.

You could always try a gravity bleed at home and see if maybe the dealer just didn't bleed them completely.