Merry Xmas. Yep, I've had brake failure twice in the Fluence.
Both times were after driving through pools of standing water and getting the red brake warning sign on the dash. If you ignore the warning and carry on (knowing that the sensors are flooded with the drenching) your brakes are fine for about 20 mins. But then watch out - you suddenly aint got hardly any brakes at all. I've not had the feeling of acceleration you describe - just no brakes.
Once everything has dried out all is back to normal - the moral is take the red brake warning light very seriously - I have about 20 mins grace after it comes on, and both failures have been sub 7 mph. Previous posts seem on the money there. But scary - yes, you just keep rolling forward.... I reached for the handbrake on both occasions.
Never mind- still love the Fluence (hec, I must do...) and now on 19,000 miles.
All best to you all for 2015, keep the faith,
Chris (number 6, or was I 5?