Many vehicles now have a concentric slave cylinder (CSC) instead of the traditional release fork and bearing:

The CSC is usually mounted inside the gearbox bell-housing and is made of modern thermoplastics or aluminum, with intricate rubber seals and chambers within. When a clutch is replaced, it is critical that the CSC is also replaced. After a number of clutch actuation, the rubber seals wear out and the operating position of the CSC changes, indicating that the worn internal components are no longer aligned. To ensure that there is no hydraulic fluid loss, the rubber seals inside CSCs must be of the highest quality. Allowing fluid to pass through the seals contaminates the friction material on the disc, resulting in clutch failure. The CSC should not be compressed before fitting because it will damage the rubber seals because they are not fully lubricated.