All of the Mercedes Benz cars since the early days have a brake booster hose that runs from the brake booster to the intake manifold; or if it’s a diesel, to the mechanical vacuum pump. Inside this line there is a check valve made of plastic which is exposed to allot of heat under the hood. Over time it becomes brittle and can break. This is a major safety concern because if the valve breaks the brake pedal will go as hard as a rock and the car will hardly stop. When we are servicing your car we always check these lines and their condition, and recommend replacing it if necessary. The cost of this repair is less than $100.