Ever notice the clutch slipping, failing to fully engage, or mashed the clutch pedal down only to feel it go straight to the floor? Any of those signs can mean that air has infiltrated your clutch system.
Getting rid of it is key for restoring proper clutch function and smooth gear changes. This step-by-step guide covers how to properly bleed a clutch master cylinder.
How the Clutch Master Cylinder Works
Your car’s clutch master cylinder acts as a mediator between the pedal and the mechanical clutch. When you press the clutch pedal, it converts that motion into hydraulic pressure to disengage the clutch.
Inside is a piston and spring assembly. Pressing the pedal pushes the piston, compressing the spring to create pressure.
That pressure then flows through a brake fluid-filled hose to the slave cylinder. The slave cylinder receives the pressure and disengages the clutch so you can shift gears smoothly.
Essentially, the master cylinder enables this process:
- Press pedal
- Master cylinder builds pressure
- Pressure transfers through fluid line
- Slave cylinder gets pressure signal
- Clutch disengages
Over time, worn seals can lead to leaks and a spongy, ineffective clutch pedal feel. This may require bleeding the system or replacing the master cylinder.
7 Steps to Bleed a Clutch Master Cylinder
Tools and Materials Needed
To allow the process goes smoothly, gather the following items:
- Floor jack and safety jack stand to raise your vehicle safely
- A set of wrenches: a brake bleeder wrench or line wrench (commonly 8mm or 10mm)
- Aquarium tubing: one to two feet to connect to the bleeder nipple and a vessel to catch the old fluid
- An empty water bottle or similar container to hold the expelled fluid
- Rags for cleaning up any spills
- Gloves and safety glasses to protect your hands and eyes
- Hand-operated vacuum bleeder or suction tool for an easier bleeding process (optional)
- Brake fluid: generally DOT 3 or DOT 4. Consult your owner’s manual to determine the correct type for your specific vehicle
Keep in mind that some people prefer using a vise and caliper to stabilize the clutch master cylinder, but this is not necessary for everyone.
1) Prepare to Bleed the System
Park your car on a level surface. Make sure your engine is turned off and engage the parking brake. For added safety, you may want to use wheel chocks as well.
The first thing you need to do is locate the clutch slave cylinder of your vehicle. If you know where your clutch fork is on the transmission, the clutch slave cylinder should be attached to it.
Once you’ve found the slave cylinder, locate its bleeder valve. This should look like a small nut on the end which usually has a diameter of about 8mm. The nut is often covered by a rubber nipple to keep dirt and debris out.
A good setup for bleeding is a tube that will fit over the end of the bleeder screw and provide a decent seal. It doesn’t need to be perfect, just enough to get air and hydraulic fluid out of the system and into a container. Run the other end of the tube into a drain pan, bucket, or small container.
2) Open the Bleeder Valve and Purge Air
Ask your partner to apply firm pressure to the clutch pedal in the cabin of your vehicle. Tell them to press down the clutch pedal all the way until it reaches the floor.
While they’re pushing down on the clutch pedal, you will use a wrench (usually 8mm) on the slave cylinder bleeder valve to loosen it up.
Keep loosening the valve until air comes out. Moments later, you should start seeing hydraulic fluid “bleeding” out after that. Make sure you place a container down so that the fluid can fall into it.
Note that it is best to use a flare nut wrench for clutch and brake bleeding. These wrenches are designed to put as much surface area as possible on the bleeder screw so you minimize the chance of stripping the nut.
3) Close the Bleeder Valve
Use the wrench on the bleeder valve to tighten it back up again while the clutch pedal remains depressed. Releasing the clutch before the bleeder screw is tightened could suck air back into the system.
Once tightened, have your partner slowly take pressure off the clutch pedal until it reaches its regular position. See if there is still enough fluid in the master cylinder. Add more as needed.
If the fluid level drops after the bleeder screw has been tightened, you may have a leak somewhere in the system. For instance, in a clutch line, master cylinder, or slave cylinder.
Even if there is a leak the size of a pinhole, you will need to address the issue. Most parts will be replaced entirely, but some clutch master and slave cylinders can also be rebuilt to save a bit of money.
4) Repeat Until No Air Remains
Repeat steps 2 and 3. Again, have your partner apply firm pressure to the clutch pedal so that it is touching the floor. Use the wrench to loosen the valve until air bleeds out.
Keep repeating this cycle of air bleeding until there is no more air coming out, but only fluid instead. At that point, close/tighten the bleeder valve.
5) Top Off Clutch Fluid Reservoir
Have your partner slowly release the clutch pedal until it is returned to its normal position. Go to the master cylinder and open the top of the brake fluid reservoir.
Take your extra hydraulic fluid and pour it into the reservoir until the fluid reaches the “Full” line. Try not to add so much that it overfills.
6) Test Clutch Pedal
Now just test the clutch by starting the vehicle and stepping on the clutch pedal yourself. There should be a firm feel to the clutch. Also, change gears to see if they’re smooth or slippery. If they’re smooth, then everything is fine now.
7) Clean Up
Clean your work area and all the tools and containers in it. If there is hydraulic fluid on any chrome trim or paintwork, then you need to clean it up immediately because the fluid can damage the chrome and paint.
Here’s a good video demonstrating the process of bleeding a hydraulic clutch:
Dealing with Problems
There might be instances when you’re attempting to bleed your clutch master cylinder and things don’t go as smoothly as planned. This could range from persistent air bubbles in the system to ineffective clutch performance even after bleeding. In such cases, it’s important to recognize when to consult a mechanic.
When to Call a Mechanic
While bleeding a clutch master cylinder can be a DIY task, there are situations where a certified mechanic should be contacted:
- Persistent air bubbles – If air bubbles continue to appear even after several bleed cycles, it might be an indication of a more significant issue within the hydraulic system.
- Leaking fluid – Examine the system for any leaks. If you notice brake fluid leaking from the clutch master cylinder or any other components, it’s time to call a mechanic to assess and fix the problem.
- Unresponsive clutch pedal – If your clutch pedal still feels spongy or is unresponsive after bleeding the system, there might be underlying problems, such as a worn clutch or damaged master/slave cylinder.
- Difficulty shifting gears – When your car is experiencing issues with gear changes even after bleeding the clutch master cylinder, it’s likely that you’ll need professional help to diagnose and solve the problem.
Frequently Asked Questions
How Do You Know If There’s Air in Your Clutch Master Cylinder?
There are a few signs that may indicate air trapped in your clutch master cylinder. These include:
- A soft or spongy clutch pedal
- Difficulty shifting gears, particularly when downshifting
- The clutch pedal not returning to its normal position after being depressed
- A decrease in hydraulic clutch fluid
If you experience any of these symptoms, it’s a good idea to check your clutch master cylinder and consider bleeding the system to remove any trapped air.
Should the Bleeding Cap Be On or Off While Bleeding a Clutch?
When bleeding a clutch master cylinder, the cap on the clutch fluid reservoir should generally be off. This allows any trapped air to escape and makes it easier to refill the fluid during the bleeding process.
Be sure to cover the opening of the reservoir with a clean cloth or rag to prevent any contaminants from entering the system. Keep an eye on the fluid level during the process, and top it off as needed to maintain an adequate level.
What If the Clutch Hydraulic System Won’t Fully Bleed?
- Check for leaks – Make sure there are no external leaks in the clutch slave cylinder, master cylinder, or hydraulic lines. Any leaks will let air back into the system and make proper bleeding difficult.
- Bleed in reverse order – Instead of the usual order, try bleeding the slave cylinder first, then the master cylinder. Gravity can help pull more fluid through.
- Repeat bleeding process – It may take multiple bleeding sessions, manually pumping the pedal and opening bleed valves, to purge all the air out of the system. Be patient and systematic.
- Bench bleed the master – Removing it from the car to bench bleed can help get all the air out of the master cylinder itself before installing again.
- Use a vacuum bleeder – More advanced vacuum bleeders use suction to pull fluid through the system, being highly effective at purging air bubbles.
- Replace hydraulic parts – If you’ve tried everything and the system still isn’t fully bleeding, there may be an internal leak or failure in a hydraulic component that needs replacement.
Getting all the air out can seem impossible but following these tips methodically can help fully bleed even difficult clutch hydraulic systems. Reaching out to a professional mechanic is also smart if you’ve hit a wall.
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Too late for this I’m sure, but you absolutely can do it by yourself just go buy a mighty vac or a brake bleeder kit, you can even rent them at the autoparts store. $30 in comparison to the bill the mechanic will give you trust me its worth it.
My dad has an old truck in the barn that he has used in months. The clutch master cylinder of the car need to bleed out, and my dad asked me if I could do it for him. I like that you said I would need a partner to properly do it because someone needs to hold and release the clutch pedal while I take a look at the master cylinder. Since I have no one to help me, perhaps I should just take it to auto service for a clutch repair. Thanks!