In a booster brake system in a motor vehicle, the energy required to generate the braking force comes from at least one energy supply device and—unlike the situation with a power-brake system—the physical force of the driver of the motor vehicle. To convert this energy, a vacuum servo brake connected between the brake pedal and the main cylinder in a serial or tandem arrangement is typically used. The vacuum servo brake has a working cylinder which is brought into operative connection with the main cylinder via an output member and separates a vacuum chamber from a working chamber. The vacuum servo brake furthermore has a control valve which is brought into operative connection with the brake pedal via an input member and selectively connects the vacuum chamber and the working chamber to one another or the latter to the outside. In motor vehicles having an internal combustion engine, the vacuum chamber is joined to the inlet manifold of the engine. In motor vehicles having a diesel engine, the vacuum is generated by a vacuum pump driven directly by the engine.
When the brake system is actuated in normal operation, the working chamber is connected to the outside via the control valve so that air at atmospheric pressure flows into the working chamber and a mixed pressure is established there. The force applied to the main cylinder by way of the brake pedal is in this case amplified by the force arising from the difference in pressure in the chambers, with the braking pressure built up in the main cylinder as a result of this force being transmitted back to the brake pedal by way of an elastomeric reaction member which is arranged in the vacuum servo brake between the input member and the output member. If the force applied to the brake pedal now increases continuously, the mixed pressure in the working chamber increases to atmospheric pressure, at which the maximum difference in pressure between the chambers prevails. An increase in the booster braking force is no longer possible; in this condition, also described as the trigger point of the vacuum servo brake, the working chamber of the vacuum servo brake is exhausted. A further increase in the braking pressure is only possible by way of an increase in the force applied to the brake pedal. When the brake pedal is lifted, the air at atmospheric pressure is drawn out of the working chamber via the control valve and the vacuum chamber, and the working piston returns to its original position, with the braking pressure in the main cylinder being let down. If the vacuum fails, only the force applied by way of the brake pedal acts on the main cylinder.
Vacuum servo brakes of this kind typically have a substantially constant amplification ratio, so that in the operating range of the vacuum servo brake, that is to say between the condition described as a sudden change in pressure, in which the input member connected to the brake pedal comes into abutment with the elastomeric reaction member over its entire surface and the servo support of the vacuum servo brake begins, and the trigger point of the vacuum servo brake mentioned above, the curve of the output force of the vacuum servo brake emitted by means of the output member or the braking force established in the main cylinder over the input force applied to the brake pedal is substantially linear. This substantially linear curve of the output force over the input force, constant in the operating range of the vacuum servo brake, is also known as a single rate characteristic.
It has been found that when vacuum servo brakes of this kind are used, the majority of drivers cannot achieve the full booster support of the vacuum servo brake even when they believe they have the brake full on. Tests have shown that the input force the driver applies to the brake pedal is often insufficient to allow the vacuum servo brake to reach its trigger point or is reduced too early so that the maximum available booster support is not fully utilised. Even pressing the partially depressed brake pedal again no longer improves the braking procedure in that case. As a result, not all the maximum possible braking pressure is applied, so that it is not possible to achieve the best possible braking distance. Even a motor vehicle with an anti-lock braking system (ABS device) cannot be decelerated in optimum manner if the driver does not depress the brake pedal decisively enough, as a result of which the braking distance is lengthened by a few meters which are often critical.