Our 2006 vw beetle tdi, without warning, suffered a sudden reduction of braking ability due to a crack in the vacuum supply hose to the brake booster. The car became dangerously difficult to stop. Luckily we were in the flatlands, not in hilly terrain. The component is an assembly comprised of a short, straight section of rubber hose, a check valve and a longer section of plastic tubing with a 90 degree bend. This bridges between the vacuum pump on the engine and the brake booster on the firewall. The plastic tubing section takes the brunt of the vibration and flexure. Apparently, the plastic tubing becomes embrittled over time from engine heat and eventually cracks. This problem seems to be common to multiple vw models according to web forums that led me to find the problem with my own car. On the tdi (diesel) model, the loss of vacuum is not indicated by poor engine performance because the vacuum source is a pump, not engine vacuum.