Clutch fluid is a viscous oil that is used to operate a clutch in an automobile or other vehicle with a manual transmission. The fluid is poured into a tank or unit called a master cylinder, which pumps fluid through the clutch assembly. This assembly is responsible for moving the clutch during operation of the transmission. This fluid can be used in cars, trucks, motorcycles, heavy machinery, and any other vehicle that requires shifting between gears in a transmission.
Many people believe that clutch fluid is its own specifically designed fluid that is meant for use exclusively in clutch systems, but this is not the case. It is almost always brake fluid used in a different context. Brake fluid is measured based on its heat tolerance; the range is usually measured on a 2-5 scale. The Department of Transportation (DOT), is responsible for the rating system in most of North America, so brake fluid and clutch fluid are measured as DOT 2, DOT 3, DOT 4, and DOT 5 in those countries. Clutch fluid is often rated DOT 3 or DOT 4, since it does not require an especially high heat tolerance. Similar measurement systems are used in other countries to measure the same heat tolerance.
Clutch fluid is thicker than water, making it an ideal choice for hydraulic systems that rely on pressurization for proper operation. When the clutch pedal is depressed, it pressurizes the fluid in the master cylinder, which in turn pushes the fluid toward a slave cylinder mounted in or near the transmission. The slave is then pressurized, thereby moving the components within the transmission that need to be moved in order for effective shifting to take place. When the clutch pedal is released, the system is depressurized, completing the shifting process.
If any component within the clutch system fails, shifting may be difficult or impossible. It is likely that clutch fluid will leak out of the system, allowing air to get into the system and adversely affecting clutch operation. The components will need to be repaired or replaced to allow the clutch to work properly again. The fluid in the system will also need to be flushed out and replaced occasionally, especially if the system is somehow compromised during its operation. Build-up of dirt or grit can affect clutch performance, and any time air is allowed to enter the system for any reason, bleeding will be necessary. This is especially true when a new clutch system is installed; the entire system will need to be bled to ensure no air is present in the fluid.