(Updated on July 17, 2020)
The clutch master cylinder is a component that you’ll find in a manual transmission vehicle. Remember that manual vehicles require the driver to operate a clutch when stopping or changing speeds.
The purpose of the clutch master cylinder is to generate hydraulic power which allows the clutch to function properly.
Basically, brake fluid is stored in the reservoir of the clutch master cylinder. There are hoses which connect the clutch master cylinder to the clutch slave cylinder.
As soon as you step on the clutch pedal, brake fluid in the reservoir of the clutch master cylinder starts flowing to the clutch slave cylinder. This creates the hydraulic pressure needed to activate the clutch and disengage the shafts of the engine and wheels.
If you ever notice the clutch slipping or failing to fully engage, you may need to bleed the clutch master cylinder to fix this problem. This means you’re letting air “bleed” out of the cylinder.
To properly bleed the clutch master cylinder, you’ll need a partner to reduce pressure on the clutch at the appropriate time. You’ll also need more hydraulic fluid readily available. This can be done without a partner if you have a special brake bleeder kit.
7 Steps to Bleed a Clutch Master Cylinder
Below are the steps to bleed the clutch master cylinder.
1) Prepare to Bleed the System
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: