A slave cylinder is a component of a hydraulic system that works in conjunction with a master cylinder to convert force into hydraulic pressure. The slave cylinder is located at the other end of the hydraulic system and is responsible for converting the hydraulic pressure back into mechanical movement. In automotive engineering, slave cylinders are commonly used in brake and clutch systems.

In a clutch system, the slave cylinder is responsible for disengaging the clutch from the engine by moving the pressure plate when the clutch pedal is pushed in. The slave cylinder actuates the clutch fork to disengage the clutch friction plate from the flywheel, with a return spring reversing the process.

In a brake system, the slave cylinder is located inside brake calipers and/or drum brakes and is responsible for pushing the brake pads towards a surface that rotates with the wheel, creating friction against that surface. The slave cylinder found at each set of brake calipers on a car is used to close the brake pads around the brake disc.

Slave cylinders are typically mounted on the outside of a manual transmission, while others are inside the transmission bell housing. Slave cylinder failure can occur due to leaks in hydraulic fluid leading to a lack of pressure, which can make gear changes difficult to perform.