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Clutch Hydraulics

The hydraulic clutch release systems are based on the same principle as hydraulic brakes. The hydraulic system consists of a transmitter (clutch master cylinder) a hose and a receiver. The clutch master cylinder is connected to a plastic or a metal tank filled with hydraulic fluid. The tank is either common between the clutch and the brake master cylinders or separate.The receiver, depending on the application, can be a clutch release cylinder (semi-hydraulic system) or a concentric slave cylinder (full hydraulic system).


Why hydraulics?

The mechanical clutch release system consists of a release bearing that is operated by a fork. The fork is attached to a clutch cable operated by the driver’s foot via the clutch pedal. Cars have become lighter and more compact. Car manufacturers demand reduced pedal load linked to comfort reason.

Hydraulic systems become mandatory in such cases as they also provide benefits compared to the manual system, increasing efficiency of the actuation with less space required. For these reasons, one vehicle out of two is equipped with a hydraulic system today.


Benefits of hydraulic technology:

  • more flexible and reliable compared to cable systems
  • enable an optimal and constant pedal effort
  • used material is much lighter -up to 70% reduction of weight compared to the standard clutch system command -  as well as much more compact.

The hydraulic systems benefits lead the car markers to implement that technology on OE projects - one vehicle out of two is equipped with a hydraulic system today. 


Valeo Tip

In almost all cases, the clutch fluid is the same as the brake fluid. However, if the reservoir is different, see your car owner’s manual to find out what grade of fluid your car requires right image.

Discover Valeo Brake Fluid range




CRC is very easy and quick to replace as it is always located outside the transmission housing (about 30 min replacement for professionals).

Semi-hydraulic / Full hydraulic

The new clutch master cylinder generation with sensor is commonly used on applications equipped with electronic hand brake, cruise control or start & stop function.

Full hydraulic

Advantage of the CSC is to eliminate the fork and the release bearing benefiting inweight reduction by 70%.


The clutch release cylinder (CRC) may need to be replaced if:
  • The push rod does not return correctly when compressed right image
  • There is a leak from the surface


The clutch master cylinder (CMC) may need to be replaced if:
  • The clutch pedal slowly sinks to the floor and does not return
  • There is a leak from the surface

If you change the clutch master cylinder, it is recommended to change the slave cylinder (CSC) at the same time, as both units typically fail around the same time. 

The concentric slave cylinder (CSC) may need to be replaced if:
  • The clutch pedal does not return
  • Noise in cold engine and neutral position; noise disappears once the pedal is pressed:
  • Extra stroke; piston goes over its stopper
  • Oil contamination

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