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When to replace Hydraulic Components

When to replace Hydraulic Components

Why Hydraulics?

The Clutch is the interface between the engine and the gearbox. This interface can be disengaged via a release system connected to a clutch pedal to enable gear changes. In the engaged position the disc is squeezed between two friction surfaces of the flywheel and the pressure plate of the clutch cover.  To disengage a push or a pull type clutch it is necessary to push or to pull the fingers for the diaphragm. The pressure plate then moves against the flywheel and releases the friction. The clutch is then disengaged. This release system can be either mechanical ( by clutch cable) or Hydraulic

Replacement of the hydraulic components

The actuation system is composed of a mechanical part, which is the pedal, the firewall and the overcenter spring. The hydraulic system consists of a transmitter (clutch master cylinder), a hose with or without a damper and a receiver (clutch release cylinder or concentric slave cylinder). It is important to realize that the performance of the hydraulic system is also linked to the mechanical part. During each clutch replacement or once an actuation problem is detected, it is important to check all of the components in the transmission line, from the pedal to the gearbox. 

This will avoid unnecessary replacement of expensive components and unexpected labor costs. However, many clutch failures are also linked to poor functioning of the hydraulic actuation system.

Clutch Master Cylinder ( CMC)

Replace the CMC if:

  1. The clutch pedal slowly sinks to the floor and does not return
  2. There is a leak from the surface of the push rod coming out of the piston or from the connectors (Image).
  3. In such a case, the clutch master cylinder may also leak fluid inside the car.
  4. Check also any trace of hydraulic fluid behind the pedal area

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

Clutch Release Cylinder ( CRC)

Replace the CRC if:

  1. The push rod does not return correctly when compressed right (image)
  2. It is difficult and not recommended to measure effective stroke of the receiver rod as the stroke of the piston is different for each application.
  3. There is leakage from the surface of the rod coming from the piston or from the connectors


Clutch Slave Cylinder ( CSC)

Concentric slave cylinder (CSC) is positioned within the gearbox housing.Thus it is mandatory to dismount the gearbox.Replacement of the CSC itself is time-consuming (like with a clutch replacement) and labor costs are high.It is recommended to change the CSC during each clutch replacement.For many applications, the replacement of the CSC is even obligatory. 



Replace the CSC if:

  • The clutch pedal does not return: 
  • Bleed the system of air.
  • After bleeding check by applying pressure on the pedal first slowly and then rapidly to see if the fault will continue.
  • If it does, check for hydraulic fluid leakage under the chassis around the gearbox.
  • In case of leakage, proceed by removing the gearbox from the engine and check if there is leakage from the CSC If contaminated, replace the CSC.
  • Noise in cold engine and neutral position; noise disappears once the pedal is pressed:
    • This may be a sign of a relative rotation between the CSC bearing and the clutch diaphragm. 
  • Extra stroke; piston goes over its stopper
    • Press and release the bearing. If it does not move, the CSC must be replaced.
  • Oil contamination
    • Replace the CSC and check for the source of contamination
    • Repair is not possible!
    • If replacement is necessary, only new parts should be used to avoid any failure linked to the hydraulic system.
    • 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

Before or when replacing a hydraulic component


  1. For a newly purchased part, always check that the metallic clips and the protection plugs over the connectors are present in the  image.
  2. If any of them are missing, do not attempt to assemble the part on the vehicle.
  3. Do not remove the protection plugs or apply on the clips until connection.
  4. Do not pull or try to squeeze the CSC when the protection plugs are connected.
  5. There is no liquid in the hydraulic chamber and the pressure generated may cause damage / further leakage from the CSC 
  6. The connectors do not require any lubrication, so never use grease or oil to lubricate the connectors before installation
  7. It is recommended to keep the dismounted plugs for further usage. If, for any reason, it is required to dismount the hose from the CMC or CRC/CSC, the connector caps should be reinstalled to protect the connectors from oil contamination
  8. Always connect the hydraulic components to the connectors without excessive force.
  9. If CRC is replaced, it is mandatory to grease the area between the push rod and the clutch lever since, if no lubrication is applied, noise may be generated (image)
  10. Always check the bushing of the lever and change if worn out or if there is excessive free play which will impact the correct functioning of the CRC.