A clutch slave cylinder is a device used in the hydraulic clutch system; it is mounted in the transmission, either on the outside, or in the inside. If it is mounted on the outside, it is usually attached to the manual transmission by two bolts. A slave cylinder is an important part of the clutch system in a vehicles manual transmission. The slave cylinder is very important to a vehicle. A slave cylinder does not actually have to work alone; there can be multiple cylinders in a series to help with a particular performance. Hydraulic Cylinder only. Tested to assure proper operation before shipping. Clutch master cylinder.