Vacuum operatable differential servo-motor

A vacuum operable differential servo-motor for a motor vehicle having an engine intake manifold comprising:

• a housing;
• a power piston movable within said housing;
• first and second chambers positioned in said housing one on each side of said power piston;
• means for fluidly connecting said first chamber to said engine intake manifold;
• means for controlling a pressure differential between said first and second chambers including an air passage disposed between said first and second chambers and an electromagnetically operable solenoid valve for closing said air passage and admitting air to said second chamber in response to a signal;
• a failure sensing switch means for detecting operation of said power piston so as to sense any failure or unusual operation of said power piston;
• wherein said failure sensing switch means further comprises a casing disposed outside of said housing;
• a stationary terminal member and a movable terminal member engageable with each other
• a plunger for abutting said power piston and for operatively connecting said movable terminal member to said power piston in said housing;
• and
• a diaphragm seal member having a peripheral bead and a central portion formed so as to effect minimal resistance against reciprocation of said plunger, said bead being retained between said casing of said switch means and said housing, said central portion of said diaphragm seal member being sealed to said plunger.