A clutch master cylinder is a fluid pump used with a hydraulic clutch system in a vehicle with a manual transmission. It works in a similar fashion to a brake master cylinder.
When you press on the clutch pedal to disengage your manual transmission so you can shift gears, the clutch master cylinder forces fluid through a tube to a slave cylinder at the other end. Mounted on the transmission, the slave cylinder responds to input from the clutch pedal (by way of the master cylinder) to push on the clutch fork and disengage the clutch. When you release the pedal, the slave cylinder closes, engaging the clutch and forcing fluid back into the master cylinder.
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