silverfx

Hi All,
We have an issue with a Cayenne 2009 where the brake pedal seem to go in as the vehicle slows down. If you drive normal, say anywhere over 10km/h the brake operates fine, but if you slow down, the pedal goes to the floor almost (at least 2/3 in) as you need to push it further and further to slow down. Also if you stop and the pedal say is in 1/3rd, as you holding the pedal it appears that the brake looses pressure and you have to keep pressing the pedal in further and further. None of this happens if you are moving over 10-20km/h or more.
Also it only happens if the engine is running. If we stop the engine and press the pedal, it is hard and doesn't go in regardless of how long you holding it pressed.
We have replaced the ABS controller, basically achieved nothing else than now it is also complaining about the PSM, otherwise the symptoms are the same and behaves exact same.
the issue started intermittently a while back, it happened a few times before, but it became permanent since the front discs and pads were replaced about a month or two ago
Any idea?
TIA
Tom

Petza914

Sounds like a failing master cylinder allowing fluid past the internal seal.

silverfx

Thanks, but I am still puzzled, why would that not leak when the engine is stopped? The pedal should go down slowly I would think.
Pedal appears hard and set without the engine running

Petza914

Because you don't have any power brake assist when the engine is off as it's not generating any vacuum to the booster.