Patent ID: 7159704

Claim:
An hydraulic system in a motor vehicle for operating a friction clutch, the friction clutch having a clutch disk, a friction plate, a pressure plate and a spring plate, the clutch disk being securable via an action of the spring plate and mounted to rotate with a transmission input shaft of a transmission of the motor vehicle, the clutch disk being situated between the pressure plate mounted to rotate with a crankshaft of an internal combustion engine and the friction plate, a mechanical disengagement system being connected to the spring plate for disengaging and engaging the friction clutch, the hydraulic system comprising: a slave cylinder axially acting upon the spring plate via the mechanical disengagement system for disengaging and engaging the friction clutch; a master cylinder operated by a driver or automatically by an actuator system, the master cylinder having a housing and acting upon the slave cylinder; and the master cylinder including an at least two-stage spring device having a first stage and a second stage, the first stage compensating, at least partially, for a decreasing characteristic of the spring plate over the clutch disengagement path, and the second stage returning the master cylinder to a normal position with the friction clutch being engaged, the spring device including a single helical compression spring with a first section of a constant first diameter and a second section of a constant second diameter, the first diameter being different than the second diameter.