Preventing malfunctions of the clutch actuation system by bleeding it correctly


Even though the electrification of vehicle components is progressing inexorably - the electric power steering system just being one example - especially three components continue to be actuated hydraulically: the brake, the hydraulic or electrohydraulic steering system and the hydraulic clutch actuation system. What all systems have in common is that they need to be free from trapped air in order to function flawlessly.

Air as cause of malfunction


Unlike liquids, air can be compressed. As the aforementioned vehicle systems work according to the hydraulic principle, where power is transmitted by means of a non-compressible liquid column, air is the main cause of malfunctions here. As far as the hydraulic clutch is concerned, the following symptoms may occur:

  • Change of the pedal travel
  • Difficulties in disengaging the clutch
  • Imprecise pedal feel

Hydraulic release systems must therefore be bled according to the vehicle manufacturer’s instructions after clutch repairs or after a replacement of hydraulic components.

Bleeders for hydraulic clutch actuation systems

The right tool


To perform this work quickly and effectively, the relevant workshop equipment must be available. A suitable bleeder is key here. Bleeding the hydraulic clutch components without these devices is tedious and does not always produce the desired results. A collecting vessel for the hydraulic fluid will also be required, with a clear tube that fits on the bleeder nipples. Some bleeders feature a vacuum unit to level the fluid level in the compensating reservoir at the end of the bleeding process.

Clutch actuation systems


The various clutch actuation systems function on the basis of master and slave cylinder. The master cylinder is actuated by the clutch pedal, the slave cylinder uses the transmitted power to mechanically release the clutch.

The systems differ in the design of their individual components, though. The master cylinders are provided with a separate liquid reservoir, or they are fed by a line from the compensating reservoir.

The slave cylinders, on the other hand, are either designed as simple pressure cylinders actuating a lever system to release the clutch or act as concentric slave cylinders directly on the clutch pressure plate. The abbreviation CSC is also used when referring to concentric slave cylinders.

Bleeding process in individual steps


Only use liquids released by the vehicle manufacturer! Ensure absolute cleanliness when performing the bleeding process!

  • Remove filler cap from master cylinder or compensating reservoir and connect bleeder using a suitable adapter.
  • Pressurize the hydraulic system (max. 2 bar).
  • Remove protective cap from bleeder nipple on slave cylinder.
  • Attach suitable collecting vessel with clear tube to bleeder nipple.
  • Open bleeder nipple (fixed or twistable).
  • Close bleeder nipple again once the liquid in the tube no longer contains air bubbles.

For more information on the different variants of fixed and twistable bleeder nipples, refer to Service Information 12404, Bleeding Variants for Hydraulic Clutch Actuation Systems.

  • Release pressure from bleeder.
  • Disconnect bleeder adapter from master cylinder or compensating reservoir.
  • Check fluid level in compensating reservoir and correct, if necessary (observe MAX mark).
  • Place filler cap back on reservoir.

Actuate the clutch pedal several times to perform a function test. The clutch pedal must always independently return to its starting point.

Bleeding process in individual steps


Only use liquids released by the vehicle manufacturer! Ensure absolute cleanliness when performing the bleeding process!

  • Remove filler cap from master cylinder or compensating reservoir and connect bleeder using a suitable adapter.
  • Pressurize the hydraulic system (max. 2 bar).
  • Remove protective cap from bleeder nipple on slave cylinder.
  • Attach suitable collecting vessel with clear tube to bleeder nipple.
  • Open bleeder nipple (fixed or twistable).
  • Close bleeder nipple again once the liquid in the tube no longer contains air bubbles.

For more information on the different variants of fixed and twistable bleeder nipples, refer to Service Information 12404, Bleeding Variants for Hydraulic Clutch Actuation Systems.

  • Release pressure from bleeder.
  • Disconnect bleeder adapter from master cylinder or compensating reservoir.
  • Check fluid level in compensating reservoir and correct, if necessary (observe MAX mark).
  • Place filler cap back on reservoir.

Actuate the clutch pedal several times to perform a function test. The clutch pedal must always independently return to its starting point.