Vacuum servo unit for a vehicle braking system

A vacuum servo unit for a vehicle braking system comprising:

• a housing in which is defined a pressure chamber;
• a movable wall member provided in said housing to divide said pressure chamber into a constant pressure chamber adapted to communicate with a negative pressure source and a variable pressure chamber adapted to selectively communicate with atmosphere or with the negative pressure source;
• a movable power piston integrally connected with said movable wall member;
• an input member provided in said movable power piston and axially movable in response to brake actuation;
• an output member outputting a force in response to movement of said movable power piston accompanying movement of said movable wall;
• a control member for selectively communicating said variable pressure chamber with one of the negative pressure source and the atmosphere according to movement of said input member against said movable power piston;
• an operation member for applying an axial force to said output member when electric power is supplied to the operation member;
• a first input rod having one end portion connected with a brake operation member and an opposite end portion;
• a second input rod having one end portion connected with said input member and an opposite end portion connected with the opposite end portion of said first input rod, the second input rod being constantly disposed in coaxial relationship to the first input rod and being displaceable relative to the first input rod, and said second input rod transmitting force between said input member and said first input rod;
• an electric switch member provided on one of said first input rod and said second input rod, said electric switch member being operable between different states;
• a switch operation member for changing the state of said electric switch member according to movement of said first input rod relative to said second input rod.