How Often Should Clutch Fluid be Changed?
Changing clutch fluid depends on many factors, including environment, climate, and time. Clutch fluid is the same material as brake fluid, and the clutch and brake fluids should be on similar schedules for change.
Since the fluid is hygroscopic (attracts moisture), it must be changed more often in humid coastal areas rather than in a dry desert environment. Any time you observe cloudy or milky looking rather than clear clutch fluid, it is time for a fluid change and system flush to remove any moisture.
If the fluid is dirty or rusty looking, that indicates that corrosion or rust has started and may mean parts need to be disassembled, cleaned, and reassembled. If the damage is severe, then component replacement may be required. Refill with clean new fluid and bleed any air from the system so the clutch will work.
Clutch System Overview
A clutch system consists of a master cylinder mounted on the firewall under the vehicle's hood, connected to the clutch pedal by mechanical means, and a slave cylinder by metal piping and rubber hose.
The slave cylinder is mounted externally near the clutch and actuates the clutch through a clutch arm and throw-out bearing that disengages the clutch when the clutch pedal is depressed.
Another common design places a slave cylinder on the front of the transmission coaxially with the input shaft that directly acts with the throw-out bearing to disengage the clutch.
The clutch pedal travels several inches, and the throw-out bearing only travels a few thousandths of an inch, thus increasing the mechanical force several thousand times.
The master cylinder and slave cylinder are connected with a combination of metal piping and rubber hose to transfer the fluid from the master cylinder to the slave cylinder when the clutch pedal is depressed and back to the master cylinder from the slave cylinder when the pedal is released. This arrangement forms a hydraulic unit that uses brake fluid as the power transfer medium.
The design of the components of a clutch system is to block moisture. The master cylinder has a rubber diaphragm just under the lid or cap that is a combination gasket and moisture barrier. The slave cylinder is sealed to block moisture penetration.
The side of the diaphragm next to the lid or cap is vented to the atmosphere, but the inner part next to the fluid reservoir can fill the space left when fluid escapes the system allowing the diaphragm to stay in close contact with the fluid preventing moisture contact.
Moisture mixing in the fluid is the main reason for changing the fluid at intervals. When topping off the fluid in a master cylinder, this diaphragm must be removed to expose the fluid reservoir so it can be filled. The diaphragm will then need to be pushed back into the lid or cap and reset for a full reservoir.
Don’t make the mistake of topping with any fluid but brake fluid, as any petroleum product (oil) will ruin the rubber components in the master and slave system and necessitate the replacement of those rubber items.
Sometimes the slave cylinder is external and can be fairly easily accessed, but in most cases, the slave cylinder is mounted at the front of the transmission and requires transmission removal for service, which can be very expensive in labor costs.
Preparing the Vehicle
Place the vehicle on a solid surface such as a garage floor or driveway. Raise the vehicle with a jack and place jack stands to support the vehicle safely, as you will need to be under it for the draining, flushing, and bleeding processes. Gather the necessary tools and equipment (bleeder wrench, flexible tubing, catch basin, fresh fluid).
Open the hood for access to the master cylinder, which is on the driver’s side of the firewall to the left of the steering column. The usual configuration of the master cylinder is a screw-on lid with counter-clockwise rotation for loosening and clockwise rotation for tightening.
On some brands of vehicles, the master cylinder is covered with a lid that is held in place with a spring clip that needs to be moved aside to get the lid off. Usually, prying the spring clip with a screwdriver will unfasten the lid easily.
Clean the area around the cap or lid to prevent dirt and debris from entering the fluid reservoir. A few vehicles have a sharing arrangement between the brake and clutch systems with both using one reservoir for the fluid.
The brake/clutch fluid used in your clutch will be either DOT3 or DOT4 depending on your vehicle. Check your owner's manual for the correct fluid to use.
Caution is advised when handling brake fluid. Use safety goggles and rubber gloves to protect yourself from the fluid. If you do have a fluid spill, it should be wiped up immediately, and the residue washed away with water to prevent skin burns and rashes. Some individuals are sensitive to the components of clutch fluid.
Additionally, do not spill the fluid on any painted surfaces of your vehicle, as it will act as a very effective paint stripper and ruin the cosmetics of your vehicle. If any fluid is spilled on your vehicle, wash it off immediately with water as it is soluble.
Bleeding Clutch System
Access for the fluid draining and bleeding operations will be under the vehicle. The master cylinder refilling will take place under the hood. For flushing and refilling the master cylinder, use only fresh moisture-free brake fluid.
Use your vehicle service manual to find suggestions for fluid change intervals and to locate the slave cylinder bleeder valve. The usual location is on the outer housing of the transmission near the front on the left (driver’s side).
Select the proper size bleeder wrench (it should be a 6-point box-end wrench type to avoid rounding the hex portion of the bleeder valve). A short piece of flexible tubing [1/4” works on most bleeders] (available at most hardware stores) placed over the bleeder valve and directed toward the catch basin can save a cleanup mess.
If you use the tubing, you will need to place your bleeder wrench on the bleeder valve before the tubing.
Place a catch basin under the bleeder valve and open it. Move to the under-hood location of the master cylinder and remove the lid or cap and the diaphragm. At this time, the fluid should run into the catch basin.
When the master cylinder reservoir is empty, pour in some fresh fluid to rinse out any old moisture-filled fluid remaining. Close the bleeder and fill the master cylinder reservoir with clean fluid, and open the bleeder until the fluid runs out with no air bubbles.
Refill the master cylinder reservoir and have a helper depress the clutch pedal several times and then hold it depressed while you bleed the clutch system at the bleeder valve. Close the bleeder valve.
Repeat this process until no air is present when opening the bleeder valve while the pedal is depressed. You can visually observe the air bubbles in the fluid and also can hear them making a slight hissing sound as they exit the bleeder valve.
All air must be bled from the lines, slave cylinder, and master cylinder as it is quite compressible and the fluid is not. The clutch system will not work properly if air is present in the system.
Be sure to keep the master cylinder reservoir filled between bleeding cycles, as allowing it to run out of fluid will introduce more air into the components requiring you to repeat the bleeding process.
If no air is present, tighten the bleeder valve and refill the master cylinder reservoir. Replace the diaphragm in the lid or cap and push it in until it is back in place for a full reservoir. At this point, you have successfully changed the clutch fluid.
Powered bleeders can be used to bleed the system. Pressure bleeder that pressures fluid through the slave cylinder to the master cylinder, thus expelling trapped air. Vacuum bleeders use a vacuum to suck fluid and air from the system through the bleeder and, by this means clear the air from the clutch system.
Fluid Change Intervals
The interval for changing clutch fluid will vary greatly between vehicles and drivers, along with environmental factors. If the system is not opened up to the atmosphere for checking purposes, it can be a long time and many miles between changes.
EXAMPLE: The author has a 23-year-old vehicle with 244,000 + miles on it and the fluid has never been checked or changed, but the clutch still works very well. The vehicle is operated in a semi-arid environment and does not get exposed to moisture very often.
The author has serviced vehicles with much fewer miles and lesser age that had moisture in the clutch fluid system and required fluid change for further operation. Sometimes the moisture problem was severe enough to cause rusting or corrosion of internal components requiring replacement.
Each vehicle is an individual case for knowing just when to change the clutch fluid.
Your owner’s manual may have suggestions for clutch fluid change intervals. If you are interested in having a definite schedule for changing clutch fluid, the author recommends changing the fluid every 3 to 5 years. If moisture is present in the fluid causing it to look cloudy or milky, then the change interval should be shortened.
Special attention should be given to the diaphragm/gasket of the master cylinder for any possible deterioration that would allow moisture to get into the fluid reservoir. This moisture can come from the humidity level of atmospheric air.
Being hygroscopic (attracts moisture), the clutch fluid will absorb moisture from any source it can come in contact with.
Most people that have do-it-yourself skills and experience and a few tools can do this job fairly easily, but if you aren’t comfortable attempting the clutch fluid change it is not a terribly expensive job to hire out to a professional mechanic.
Most mechanics will have the experience necessary to judge the extent of moisture contamination and, if indicated, can suggest checking further into the hydraulic clutch components for corrosion damage that will need to be repaired.
Changing the clutch fluid is fairly easy and a short job. It can be done in less than half a day.
Clutch Fluid FAQ
What are the symptoms and signs of low clutch fluid?
Low clutch fluid will result in the clutch not disengaging and making gear changes difficult to impossible. It will cause vehicle creep when stopped at traffic lights and make shifting into gear difficult, causing a grinding sound.
Does clutch fluid need to be changed?
Clutch fluid will need to be changed any time it has absorbed moisture. The fluid is extremely hygroscopic (attracts moisture). If the fluid is not changed the absorbed moisture can cause corrosion and rust of components, making repair or replacement necessary.
How do you check clutch fluid level?
Many master cylinders have a clear plastic strip on them that allows a visual indication of fluid level. Other master cylinder reservoirs are made of clear plastic that allows a quick visual inspection of fluid level.
Luxury vehicles may have an electronic indicator and/or alarm in the dash-mounted instrument cluster to indicate low clutch fluid. Others will require opening the cap or lid, removing the gasket/diaphragm, and visually checking the fluid level.
Do you bleed clutch fluid?
When changing clutch fluid you will need to bleed all the air from the system as it will not work properly if it contains air (see bleeding system section above).
What causes loss of clutch fluid?
The most likely cause of clutch fluid loss is a leaking slave cylinder. The next likely is a leaking master cylinder, and then the hose and piping are the final places to examine.
Do all manual cars need clutch fluid?
Only the ones that utilize a hydraulic clutch system. Some vehicles use a strictly mechanical method to make the connection between the clutch pedal and the mechanism to operate the clutch.