Pretty much all modern gasoline and diesel powered vehicles today have transmissions (although some electric vehicles do not), which allow the car to use multiple gears. A transmission capable of shifting through multiple gears affords modern vehicles several advantages. First and foremost, it allows a car to accelerate faster at low speeds while achieving higher top speeds. Similarly, there is a huge fuel efficiency improvement in that lower engine RPM can be used at a given speed.
Transmissions come in a couple common flavors: manual and automatic. Manual transmissions rely on the driver to shift gears by disengaging the motor from the transmission via the clutch, selecting the correct gear, and then reengaging the motor to continue driving. The clutch system relies heavily on the master cylinder to operate.
Function of a Clutch Master Cylinder
The clutch master cylinder is a necessary component for the clutch system to operate properly. Practically speaking, it eliminates the need for mechanical linkage connecting the clutch to the clutch pedal in favor of a hydraulic connection, which is much more flexible in terms of space claim and ease of integration from a vehicle engineering standpoint.
A clutch master cylinder is a hydraulic vessel that contains a piston. When depressing the clutch pedal, the piston in the master cylinder is actuated, which compresses and thereby pressurizes the brake fluid in your clutch system. As a hydraulic line connects the master cylinder to the clutch slave cylinder, when the fluid in your clutch system is pressurized, it imparts hydraulic force on the piston in the slave cylinder which acts to move your clutch.
In other words, the clutch master cylinder converts the mechanical energy from depressing the clutch pedal into hydraulic energy which acts on the slave cylinder. The slave cylinder converts this hydraulic energy back into mechanical energy which moves your clutch disc.
Common Symptoms of a Bad Clutch Master Cylinder
There are a number of symptoms of a failing or failed clutch master cylinder. Because such a direct connection exists between the driver and a manual transmission, these problems are usually very noticeable and can be felt readily by most drivers:
Low/Dirty Clutch System Fluid
Low fluid levels indicate a leak somewhere within your clutch system. Dirty fluid can similarly indicate a slow leak as it can allow contaminants to enter the fluid. It should be noted that low fluid is not necessarily an indicator of a bad master cylinder, as fluid can leak from the slave cylinder or the hose connection points within the system as well. As such, it’s important to verify where a leak originates before attempting a repair to ensure that you replace the correct component.
Difficulty Shifting/Impossible to Shift
Leaky seals within the master cylinder can mean that the fluid in your clutch system does not reach full pressure when you depress the clutch pedal. When the pressure at the slave cylinder is low upon clutch pedal depression, it can prevent the clutch from fully disengaging. When this occurs, the transmission can become very difficult or even impossible to shift and can sometimes cause grinding or vibration during shifts.
Light Clutch Pedal
Leaky seals that allow fluid to blow by the piston in the master cylinder upon clutch pedal depression can result in light clutch pedal feel, as the leaky seals prevent you from building full pressure within your clutch system. Sometimes this can be severe enough that it prevents the clutch pedal from returning to the top of its travel.
Replacing a Clutch Master Cylinder
Note: The example steps below are intended for general informational purposes solely to help give you an idea of project difficulty and tools required. As all cars are engineered differently, repair procedures and safety hazards vary from vehicle to vehicle. To ensure that you have a vehicle specific repair procedure and an exhaustive list of potential safety hazards, we advise you reference a factory service manual for your vehicle. Similarly, referencing a repair manual such as Chilton or Haynes might serve as a less expensive alternative.
Step 1 – Park your vehicle on a flat, hard surface and make sure the transmission is in gear.
Step 2 – Raise your vehicle using a floor jack and support it on jack stands such that you have access to the underside of the vehicle. Make sure your vehicle is stable on the jack stands before climbing underneath.
Step 3 – Remove the cap from the clutch master cylinder reservoir and use a small pump to remove the fluid from the reservoir. A turkey baster is a pretty affordable hand pump in this context (although you should not use it for any food preparation once it has been deemed an automotive tool due to the toxicity of chemicals it may come into contact with). Once done, you can replace the cap to prevent dirt or debris entering the reservoir. Be cautious not to drip brake fluid on any painted surface. It is very corrosive and will damage paint.
Step 4 – Remove the hydraulic line from the clutch master cylinder with a flare nut wrench or similar. Cover the end of the line with a plastic bag to prevent debris from entering the system and prevent leaks from occuring. Take extra care not to bend or break the hydraulic line.
Step 5 – Next go into the driver’s compartment and under the dash to remove the cotter pin that holds the clevis and push rod that attaches the master cylinder piston to the clutch pedal. You may need a small set of needle nose pliers to pull the cotter pin out. Once the pin is out, remove the clevis and push rod from the clutch pedal.
Step 6 – Remove the mounting nuts that hold the clutch master cylinder to the firewall with a socket and ratchet. Put the master cylinder in a bag to contain any fluid and retain for safe disposal after the project is complete.
Step 7 – Remove the new clutch master cylinder from its package and ensure the seal is good on the back of the cylinder housing. Check whether your new master cylinder requires bench bleeding prior to installation, or whether it can be immediately installed to your vehicle. If the master cylinder requires bench bleeding, clamp it gently into a benchtop vise and follow the manufacturer’s instructions.
Step 8 – Once the new master cylinder is ready, mount it to the firewall of your vehicle. Add the mounting nuts to secure the master cylinder, then replace the clevis and push rod under the dash and reinstall a new cotter pin. As cotter pins can wear out after being bent a few times (in the same way a paper clip can break after bending it back and forth a few times), we suggest replacing yours with a new one to avoid a potential failure while driving, which would incapacitate your clutch system And cause potential danger.
Step 9 – Reattach the hydraulic line. Be careful not to cross-thread the fastener and not to drip brake fluid anywhere.
Step 10 – Refill the clutch master cylinder reservoir with new fluid from an unopened or newly opened container, using the appropriate fluid specified in your vehicle’s owner’s manual. Once full, you will need to bleed your clutch system. You can use a brake bleeding kit for this task if you want to save yourself some time and headache. Once bled, verify that the pedal feel is correct to ensure the master cylinder has correct pressure when pushing the clutch in. It should be noted that some clutch master cylinders share the reservoir with the brake system. If this is the case, make sure your brake system is properly bled as well.
Step 11 – If you needed to lift your vehicle, lower it back to the ground. Double check that everything was installed properly, and verify your fluid level. If everything is installed and operating properly, verify the repair with a short test drive, troubleshooting as necessary.
Best Clutch Master Cylinder Brands
When it comes to selecting a replacement clutch master cylinder for your vehicle, there are many brands out there to select from. To help narrow your search, we have provided a few of our favorites as we have found them to be a good balance of quality and value over the years.
Centric Parts has a complete hydraulic component line-up that includes clutch hydraulic replacement parts, brake master cylinders, wheel cylinders, brake hoses, etc. Their attention to detail means drama free installation as sealing surfaces are flat, mounting hole tolerancing is good, and build quality is strong. We are sure that a Centric clutch master cylinder will treat you well for the long haul if you decide to use one for your next project.
Dorman offers a complete line-up of replacement master cylinders for your clutch system, meaning they have wide ranging coverage across the automotive industry so they will have a part that fits your car regardless of what you drive. Dorman delivers excellent performance and reliability due to the high build quality. The materials they use are nice, which leads to an increase in long-term durability. A Dorman clutch master cylinder is likely to serve you well for quite some time.