I have a 2005 Audi A4 Sedan Quattro 2.0T automatic VIN WAUDF68E65A43xxxx. When I first start the car, and back up or go forward, the brake pedal is soft and I have hardly any brakes. If I drive the car for 1-2 minutes, the brakes are fine. I suspected a vacuum leak and have looked in the hoses, checked around the brake booster hoses and can feel no leak or cracks or hear no hiss sounds. I disconnected the vacuum host where it goes thru the rubber grommet and the firewall joining with another hose leading to the booster after letting the car sit overnight - there was a small hiss of vacuum indicating at least some residual vacuum - I guess there would be a check valve somewhere in the vacuum lines heading from the engine to the booster to prevent loss of vacuum so brakes would be good on startup ? The car has 75K miles. No brake light, No CEL, no codes. can anyone suggest a diagnosis/fix for this issue ? Thank you.