BRAKE SERVO
A vacuum servo is a component used on motor vehicles in their braking system, to provide assistance to the driver by decreasing the braking effort.
A vacuum servo also known as a power booster or power brake unit uses a vacuum to multiply the drivers pedal effort and apply that effort to the master cylinder. ... The vacuum booster is attached between the master cylinder and the brake pedal and assists the braking force applied by the driver.

Failing Master Cylinder: If you hear a hissing sound and suspect that the problem is thediaphragm in the booster, the actual cause islikely the master cylinder beginning to fail. Signs that the master cylinder is leaking into the brake booster include low fluid in thereservoir with no visible leaks.
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