b) Brake Booster
If the legally limited control force is not sufficient both for the production of the braking force necessary for the vehicle's traffic safety as well as in terms of comfort, a brake booster is used to generate an auxiliary force FH, which superimposes this control force. Thus for the output force of the booster FO we get:
Eq. 3-63
As characteristic value of the brake booster, we introduce the boost B*:

Eq. 3-64
However, the boost has to be limited using constructive measures since the foot force required for reaching the prescribed braking retardation is not allowed to exceed a maximum value even in case of a failure of the brake booster which is a part of the transmission system. In order to achieve this, a so-called modulation point (FH 0, Fop 0) is determined beyond which the auxiliary force is not increased any further by limiting the pressure.
For Fop ' Fop 0 the boost factor is:

Eq. 3-65

with an auxiliary force proportional to the control force.
Over the proportionality factor that is valid up to the modulation point (and in it):

Eq. 3-66 As a result: Eq. 3-67

For Fop > Fop 0 the boost factor is:

Eq. 3-69

In the design of braking systems, the modulation point should be fixed in such a way that the loaded vehicle can be braked at the blocking limit using the operating force Fop 0 so that the driver does enter into the range of diminishing boost. The figure shows a usual booster diagram.