Build-up of partial vacuum

For the first stage, the largest portion of the spatial enlargement (discharging) is used, thus building up the partial vacuum quickly for the brake servo. Only in the last section is the opening released for the additional loads, i.e. the second stage is activated. Building up the partial vacuum for the additional loads takes longer, as shown in the following diagram.

Fig 1: Vacuum Supply Graph

Courtesy of BMW OF NORTH AMERICA, INC.