The manual transmission was once the transmission of choice due to its ability to operate reliably with lower cost to assemble than the average automatic. In the early days, cars were built almost exclusively with manual transmissions for this reason. In the middle of the century, automatics hit the scene and were viewed as luxury transmissions. In the last 30 years, the manual transmission has become less and less popular, perhaps due to the performance advantages of modern automatics in terms of shift speeds and improved gear ratios. In light of this, manual transmissions are now chosen for the small minority of new cars sold in the United States, and with the advent of better and better automatic transmission technology, that number is likely to continue to decline.
That said, if you have an older vehicle or prefer the engagement and connectedness of a third pedal and a shift lever over setting the fastest lap time, you may still have a manual transmission. Most modern manual transmissions have hydraulic clutch systems instead of a mechanical release system such as a clutch cable. One of the most critical components in a clutch hydraulic system is the clutch slave cylinder, which also happens to be the component that most often needs replacement.
Function of a Clutch Slave Cylinder
The function of the slave cylinder is to work in partnership with the clutch master cylinder to control manual transmission clutch engagement and disengagement for shifting gears. When you press your clutch pedal, it actuates a piston in your clutch master cylinder which pushes hydraulic fluid (usually brake fluid) through your clutch lines and into your slave cylinder. When the hydraulic fluid is pushed into the slave cylinder, the slave cylinder’s piston extends to engage the clutch fork, which pivots to disengage the clutch from the engine flywheel. Releasing the clutch pedal allows the slave cylinder piston to retract and reengages your clutch.
There are two styles of clutch slave cylinders primarily available: concentric and external. By far the most common type is the external style. The external style is mounted on the outside of the bell housing for the manual transmission. External slave cylinders typically contain a piston, internal spring, and a pushrod to move the clutch release fork and release bearing.
The second style, called the concentric style, is typically mounted inside the bell housing in place of a normal style release bearing. This eliminates the clutch fork and related hardware, offers better alignment geometry, and is a smaller unit that decreases the overall package size. Accordingly, concentric slave cylinders are used in instances where there are tight space requirements. The downside to this style is that your transmission often needs to be removed to replace the slave cylinder if it fails, which usually means big bucks or a lot of hours underneath your car.
Common Symptoms of a Bad Clutch Slave Cylinder
The hydraulic clutch system on most cars is pretty simple, and as such it is pretty easy to identify which component has failed. The following are the most common symptoms that present when your clutch slave cylinder is faulty:
Visible Leaks – Perhaps the easiest symptom to diagnose is when the slave cylinder develops an external leak that you can see during an inspection under the car. The first sign may be a puddle of brake fluid under the car on the garage floor that is directly beneath the slave cylinder. In this case, you can usually confirm by looking at the slave cylinder to see if it is wet. You may have to pull back the boot on the piston slightly. If you notice that the slave cylinder is wet, you have a leak.
Changes In Clutch Pedal Feel – Your clutch pedal should have a consistent feel over the life of the clutch slave cylinder. If the clutch pedal suddenly changes in a way that makes it feel spongy, softer, or slower to return after you remove your foot, this may indicate a faulty slave cylinder. Most commonly, the fault is a leak that has allowed air to enter the system. While hydraulic fluid is incompressible, air is not. This means that as you press your clutch pedal when air is in the system, the air compresses instead of moving the slave cylinder piston.
Difficulty Shifting Into Gear – The transmission can be hard to shift with a bad slave cylinder, meaning that it takes increased effort and you may notice increased notchiness when shifting. This happens when the clutch fails to fully disengage and drags on the flywheel, which can be the result of a faulty clutch slave cylinder.
Car Moves When Clutch Pedal Fully Depressed While In Gear – If you have a gear selected and push your clutch the floor, it is possible for your car to move if your clutch doesn’t fully disengage due to a failed slave cylinder. This is most noticeable in first gear or reverse when your car is on a flat surface. The reason for this is that the piston on a failed slave cylinder sometimes doesn’t fully extend, meaning that if your clutch pedal is pressed to the floor, your clutch is still in partial contact with your flywheel.
Low Fluid – You may notice that the fluid is low in your clutch fluid reservoir when your slave cylinder has failed. One of the most common reasons for failure is leaking, and sometimes these leaks aren’t readily visible on the exterior of the slave cylinder. In these cases, you are unlikely to see a puddle on the ground, but fluid will still leak out of your hydraulic system. In this instance, the first thing that you are likely to notice is low fluid. In addition, a leak can create a pathway for water to enter your fluid (which is designed to absorb water) which can make it dirty and reduce its performance over time.
Replacing a Clutch Slave Cylinder
If you find that you have a leak at the clutch slave cylinder, or it’s stopped operating correctly, you have to take the time to replace it correctly. Fortunately, the job is pretty straight forward. The following is a basic summary of the typical steps to replace your clutch slave cylinder intended to give you an idea of difficulty, however the exact method may differ slightly depending on vehicle make and model:
Step 1 – Park your vehicle on a flat surface with the transmission in gear and parking brake engaged to prevent the vehicle from rolling.
Step 2 – While some vehicles are high enough to allow you to access the slave cylinder without lifting, raise the vehicle with a floor jack if you need extra clearance to be able to get under your vehicle to replace the clutch slave cylinder. If you do lift your vehicle, you should support the vehicle with jack stands for safety. Since you will be working with fluid, it is wise to don a pair of safety glasses to protect your eyes.
Step 3 – Remove as much brake fluid from the reservoir as possible. You may be able to do this with a small vacuum pump or siphon, but just be sure to avoid getting brake fluid on any painted surface as brake fluid will very quickly damage your car’s paint. Once the reservoir is empty, you can slide under the vehicle. A mechanic’s creeper can make this easier. Remove the line attached to the slave cylinder using an appropriately sized flare nut wrench. It will have a minimal amount of brake fluid in the line, so place a drip pan under the slave cylinder to catch anything that comes out. You can place a plastic bag and rubber band over the end of the line to prevent dirt from entering the hydraulic system during replacement.
Step 4 – Remove the fasteners or clamps holding the clutch slave cylinder to the transmission bell housing. Usually it’s just a pair of bolts that can be removed with ratchets or hex keys found in a standard mechanic’s tool set. Then you can remove the slave cylinder to prepare for the new replacement part to go on.
Step 5 – Depending on the manufacturer’s recommendations for your replacement slave cylinder, you may want to bench bleed the new slave cylinder prior to installation to remove the air and save yourself some time and frustration. You may accomplish this using either a brake fluid pump or a long hose and gravity. Be sure to complete this step if required, but first double check that the new cylinder isn’t already pre-filled with fluid out of the box as that will eliminate this step completely if that is the case.
Step 6 – Clean the surface where the new slave cylinder will be installed, then install the new cylinder onto the transmission bell housing using the previously removed fasteners. Torque the fasteners to spec using a torque wrench. Once installed, you can remove the plastic bag off the hydraulic line and reinstall the line on the new slave cylinder, being careful to make sure the threads align before you use a tool to tighten. Typically this does not require pipe tape since the thread geometry allows for a complete seal.
Step 7 – Next you will need to bleed any air from the slave cylinder assembly that entered during the replacement or reassembly process. This can be done with a single person brake bleeder kit, or you can utilize a second person to help by pressing the clutch pedal while you open and close the bleeder screw. In the case where you enlist the help of a friend, you will need to add brake fluid to the reservoir prior to starting the bleeding process, and may need to add fluid during the process to prevent air from entering the system.
Step 8 – Once the cylinder is bled completely, tighten the bleeder screw fully and clean the area with a microfiber towel or shop rag to remove any excess brake fluid. Be sure to fill the reservoir back to the full line and replace the cap on the reservoir when done, otherwise you are likely to have a mess on your hands.
Step 9 – With the clutch hydraulic system restored, you can now lower and test drive the vehicle to ensure the system is back to operating normally. You can test shift the transmission while the car is idling before driving it and ensure the feel is restored. As always, start the test drive slowly and easily to verify the clutch system is back to operating 100%. The transmission should be easy to shift between gears without grinding and you should no longer be experiencing any of the aforementioned symptoms of a bad slave cylinder.
Best Clutch Slave Cylinder Brands
When it comes to selecting a replacement clutch slave cylinder for your vehicle, there are many brands out there to select from. To help speed your search, we have provided three brands we highly recommend as we have found them to be of consistently good quality.
Centric Parts has a complete hydraulic component line-up that includes clutch hydraulic replacement parts, brake master cylinders, brake hoses, and caliper repair kits. Their clutch slave cylinders are affordable and popular because they tend to last quite a while. Centric also offers a line of pre-filled clutch hydraulics to reduce issues to make life easier as you can skip the bench bleed. Overall, we think you’ll be happy if you choose a Centric slave cylinder.
Rhinopac is known for building consistent quality products that meet or exceed OEM specifications. Overall their quality is good, and they offer a pre-bled line of clutch hydraulics similar to Centric which means that you can save time and potentially skip the bench bleed step. Overall the prices for their clutch hydraulic components are pretty affordable, and considering that they last, they tend to be a pretty good value. They are also pretty easy to find since they are pretty popular.
Dorman offers a complete line-up for a replacement slave cylinder for your clutch system. They use high-grade materials that meet and exceed OEM spec to increase long-term durability and reduce your chance to need another replacement in the future. This is obviously good for your peace of mind, but it’s also good for your pocketbook as you won’t have to replace a faulty part anytime soon since the quality is high.