Brake pedal sinking under heavy push

[Paul Atherton]

If you have seen any of my recent posts you'll know I recently changed 2 rear callipers and flexi hoses as I had one seize on me. Ever since I did this job I noticed that If I am stationary, and push my brake pedal hard as I can, and hold it down under constant pressure (with engine running, so with servo assist), then the pedal slowly sinks to eventually hit the bump stop. Now you would never drive using the brake like this, so its not a complete show stopper, but worrying all the same. I bled the brakes thoroughly and there is no air in the system as far as I can see, and no leaks whatsoever. I even cycled the ABS whilst pressure bleeding the front callipers (using VCDS). No air detected in system at all - but the pedal still sinks if I old it down hard. So I thought to myself, maybe the master cylinder has a flipped seal, so passing fluid. So today I fitted a brand new Brembo master cylinder and again did a thorough bleed. The issue is still exactly the same - no improvement whatsoever! I know there is supposed to be an ABS bleed procedure on the A6 C6 F4 that you can access using VCDS via group 002 basic options under ABS Brakes, but this option does not work on my car - displays an error invalid group message. Instead I use group 010 to cycle the ABS whilst doing a pressure bleed.

I'm stumped now, and shy of taking it to an Audi specialist, I am out of options unless you guys have any ideas? Thanks in advance, Paul

 

[NHN]

Got the same exact issue on my A3 mate, hoping I'll resolve tomorrow if weathers ok, got a gunson pressure kit to try.

 

[Paul Atherton]

I've been using a pressure kit very similar to the gunson one from eurocarparts. I only manage to get the pressure to about 10psi before the screw on reservoir cap starts leaking. The reservoir cap is a poor fit to be honest. Hopefully the gunson one will be better quality. Let us know how you get on.

 

[Paul Atherton]

Just realised that I never posted the solution in the thread. The reason my pedal was sinking was nothing to do with the hydraulics at all. Turned out I had front rotor delamination and by applying max brake pressure, I was actually compacting the laminations of steel. The rotors from the outside looked like new (first image), but the inside surface was delaminated (second image). A real mess! New rotors and pads fitted and brakes feel like new.