Bleed the Hydraulic Clutch

Phil Krolick – Automotive

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Most hydraulically operated clutches use brake fluid for the hydraulic fluid.

Brake fluid is hygroscopic and will attract moisture out of the air.  Over time this moisture in the brake fluid will lead to corrosion inside the hydraulic system causing both the master cylinder AND clutch slave cylinder to wear. 

It is often possible to replace the clutch without getting air in the hydraulic system.  If you are replacing the clutch and not replacing the hydraulic clutch fluid you are encouraging corrosion and early failure for the hydraulic clutch system.

A common problem on older vehicles is the clutch hydraulic system will start to leak.  This will cause the clutch to drag or not fully disengage.  If the master cylinder is low on fluid it will be due to a leak.  When one component starts to leak you should expect the other unit to be close to leaking.   If either the master cylinder - or the slave cylinder is leaking I recommend replacing BOTH units for maximum reliability in the future. 

Some vehicles may use HSMO or Hydraulic System Mineral Oil 

HSMO is used in very few vehicles.  It is green in color and can not be mixed with brake fluid.  Using the wrong type of fluid will ruin the master cylinder AND the slave cylinder and require replacing all components in the hydraulic clutch assembly.

If the proper type of hydraulic fluid is not clearly stated on the master cylinder cap, ALWAYS look up the proper type fluid.  There may be other type hydraulic fluid specified other than Brake Fluid or HSMO. 

Clutch bleeding can be performed a variety of ways.  Most procedures tell you to slowly push the clutch pedal to the floor and hold it down while releasing the bleeder screw on the clutch slave cylinder.  Hydraulic fluid will then run out the bleeder along with air that may be in the system.  Close the bleeder screw before releasing the clutch pedal then repeat the procedure until all air is out of the system.  If you slowly press the clutch pedal it works much better than rapidly pumping the clutch. This method will get the air out of the hydraulic system however it will not replace all the brake fluid!


The method I prefer is vacuum bleeding.  It will bleed AND replace all the brake fluid in the clutch hydraulic system.

vavuum bleeder

To vacuum bleed you connect the clear hose to the bleeder screw on the slave cylinder.  Connect pressurized shop air to the bleeder bottle.  As compressed air blows past the top of the bleeder bottle it creates a vacuum inside the bottle.  This pulls brake fluid out of the bleeder screw.   Be sure to have the vacuum applied while you are closing the bleeder screw.  This method works great IF you do not let the master cylinder run dry.  Be sure to monitor the fluid level in the master cylinder, or use the reservoir bottle shown above.  When using the vacuum bleeder air will be sucked in past the threads of the bleeder screw.   This air does NOT get into the hydraulic line however you will see that extra air entering the vacuum bleeder bottle.  When vacuum bleeding just keep sucking fluid out the bleeder screw until the fluid runs clean.  In my opinion Vacuum bleeding is the simplest and cleanest methods for bleeding a clutch.

Always properly dispose of the brake fluid.   In our shop we drain it into the waste oil container.   Always clean and dry any bleeding equipment.   The brake fluid left on the tools will damage the paint or plastic of just about everything it comes into contact with.  Wash bleeding tools out with plenty of fresh water and completely dry everything before putting them back in their proper location.   As you gain experience you will see how much damage brake fluid can do to any painted surface (including the vehicle paint!).  Any brake fluid spill must immediately get washed with plenty of water.

Other pages in this section: ASE Refrigerant Handling Certification | Classroom Presentations | Course Information and Links | Drive Train Service | DriveTrain Safety Lab | Engine Coolant | Final Vehicle Inspection | Homework and Lab Grading | Homework Assignments | How to Ruin a Drill Bit | Login To CDX | MOPAR - CAP Certification | MVAC_Lab | NATEF Lab Activities | Shop_vehicles_for_lab | to-do | Unit 1 | Unit 2 HVAC | Unit 3 | Unit 4 Auto Tranny | Unit 5

Other sections in this website: Syllabi | Electrical Systems & Engine Performance | Applied Electrical | Mechanical Processes | HV3.297 Electrical & Electronic Systems | AU3.296 Steering Suspension & Brakes | Introduction to Engine Performance | Introduction to Braking Systems
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