We are in the process of diagnosing this 2005 Porsche Cayenne Turbo S that has a warning on the insturment cluster indicating "Brake booster failure". We have checked vacuum to the brake booster and the electric vacuum supply pump. We are suspecting the booster itself. It has a pressure / vacuum sensor and a travel sensor in it. There are no codes in the ABS system or engine system — just the warning on the dash. Access and testing is quite time consuming. Will post the fix when we find it.
This warning would come on for 10 seconds when the vehicle was started and then just the small brake warning would remain lit.
The brake booster held vacuum, pressure and travel sensors tested ok. Brake booster replacement did fix this problem but double check the vacuum line as ours was brittle and broke as we were removing the booster. Vacuum brake booster line cracking is a common problem — especially at the line just under the electric vacuum assist pump.