How to Fix a Leaking Clutch Slave Cylinder

by Jack Hathcoat

Clutch slave cylinders are bolted to the transmission. Their job is to repeatedly extend a small rod to operate a clutch fork. This critical function engages and disengages the clutch disc each time the clutch pedal is depressed and released. There are two choices in servicing a slave cylinder: overhaul or replace. In the distant past, when slave cylinder bodies were made of cast iron, overhaul was a viable option. Currently, slave cylinder bodies are aluminum, so replacement is almost always the best option.

Raise the vehicle with a jack, place jack stands under the frame and lower the vehicle onto them. Follow the hydraulic line from the clutch master cylinder--located next to the brake master cylinder--to the clutch slave cylinder.

Unscrew and remove the fluid supply line attached to the slave cylinder. Remove the bolts that secure the cylinder to the transmission.

Remove the snap ring from the cast iron cylinder located behind the rubber dust cap. Remove the bleeder valve and use low pressure, compressed air to push the piston assembly from the bore. Clean the bore with spray lube. Inspect it carefully for rust and pits. Replace it if it is heavily scarred.

Attach a small brake cylinder hone to a drill. Hone the cast iron bore. This will clean and true the bore to ensure it is perfectly round. Install a new or rebuilt piston assembly. In some cases, it may be necessary to rebuild the existing assembly with new seals. These seals are easy to install but be sure to lubricate the piston seals with clean brake fluid before assembly. Install the snap ring and rubber dust cap.

Remove the dust cap and snap ring from the aluminum slave cylinder. Remove the bleeder valve. Usually the piston assembly can be pulled out of the bore without the need for compressed air. On inspection, there are typically no apparent imperfections in the bore or noticeable failures in the seals. Unless there is obvious seal failure, where it's brittle, cracked or torn, replace the cylinder. Aluminum is weaker than cast iron, and often the cylinder bore distorts and warps over time from the high hydraulic pressures. This causes fluid to bypass the seals and the cylinder to be inoperable. If the seals have apparently failed, install a new piston, snap ring and dust cap.

Install the slave cylinder, leave the bleeder valve open and add fluid to the clutch master cylinder. When fluid begins to seep from the bleeder valve, close it. Have an assistant operate the clutch pedal several times then keep pressure against it. Crack open the bleeder valve. A mixture of air and fluid will escape. Close the bleeder valve and repeat the process until a stream of fluid without air mixed in flows out. Use a drain pan to capture any escaping fluid.

Items you will need

About the Author

Jack Hathcoat has been a technical writer since 1974. His work includes instruction manuals, lesson plans, technical brochures and service bulletins for the U.S. military, aerospace industries and research companies. Hathcoat is an accredited technical instructor through Kent State University and certified in automotive service excellence.

Photo Credits

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