I've been gloating about my brother's X5's bad master cylinder (brake pedal slowly sinks to the floor when you hold pressure on the pedal) after the other disasters (paid by an extended warranty plan, $21k in repairs on a car he paid $24K for and his X5 has less than 90k miles). So yesterday I tried the brake pedal on the Cayenne S and same damn thing! Pedal sinks slowly to the floor and won't pump up to stay firm, so it's not air in the brake lines--I did have the brake fluid replaced last summer. The "rate of sinking is maybe 20 seconds for the brake pedal to bottom out with steady pressure and it doesn't hold any longer if you pump the pedal 10 times before checking how long it takes for the pedal to sink.

Have others had to replace the master cylinder before 100K miles? I suppose it's moderately likely that BMW and Porsche use the same supplier for master cylinders.

 

Engine running or off? If off - did you try pumping it a few times. You may be pressing through the vacuum assist...

 

engine idling, air conditioning running, 104 ambient temp, and after 4 hours of interstate at 80 mph.. no change after 10 pumps. our now gone Acura had the same feel at 185,000 miles and a new master cyl. fixed that. the acura pedal was sinking a bit faster than the cayenne pedal now.

 

Looks like BMW hasn't changed their ways... We had a BMW once, and it was a similar experience to what your brother had. In all my life I have never seen a car so cheaply built (under the hood). It was by far the worst car ownership experience we have ever had. To this day we have yet to buy another BMW because of the experience we had 10 years ago. One of our neighbor's has a BMW and the flat bed is over there at least once a month picking that shxt box up.

 

Have you only had the fluid flushed the one time? Dirty fluid will wear the Master Cylinder out quicker. Depending on the year Porsche as well as most manufacturers recommend a 2 or 3 yr fluid flush interval. Most people don't do theirs that often but it should be done, dirty/old fluid wear the components (seals) out faster as well as having a lower boiling point.

 

Unfortunately, the only history I have on the Cayenne S is what the dealer did before selling the car to me 2 years ago. So I don't know if/when the brake fluid was changed previously. But the Acura mentioned earlier had the master cylinder last 185,000 miles without the brake fluid ever being changed in 20 years. And it did not show a high water content based on test strips. So I have to wonder if something in the master cylinder wasn't properly heat treated or some other process was compromised if this is indeed failure of the master cylinder