S90 L6-2.9L VIN 96 B6304S (1998)
Vacuum Brake Booster: Description and Operation
In Neutral Position (Brakes Not Applied)
The illustration shows the servo in its neutral position, in which the pushrod and valve piston are pushed towards the right by the pushrod spring. The
movement is limited by the reaction disc. In this position, the valve piston holds the valve clear of its seat in the control housing, keeping the air passage
closed and the vacuum passage open. As a result, the vacuum is the same on both sides of the diaphragm, and the latter and the control housing are held
in the right-hand limit position by the diaphragm spring. When the brake pedal is operated, the rear pushrod and valve piston are displaced towards the
left (forward). Under the action of the valve spring, the valve disc is also displaced until it seats in the control housing, closing the connection between
the front and rear sides of the diaphragm. As the piston continues to move forward, the pressure is transmitted to the master cylinder, through the reaction
disc and front pushrod.
