Clutch Hydraulics: Master and Slave Cylinders

On most modern cars, the clutch pedal doesn’t pull a cable — it actuates a hydraulic master cylinder that pressurises fluid (usually the same brake fluid) to a slave cylinder at the gearbox bellhousing. The slave then pushes the clutch release bearing to disengage the clutch. The hydraulic linkage is precise and low-effort but has its own failure modes.

Master cylinder failure

Internal seal failure — fluid leaks past the piston seal, pedal feels soft or "drops to the floor." Often progressive over weeks. External leak — fluid weeps from the back of the master cylinder, sometimes onto the brake pedal area or down into the footwell.

Slave cylinder failure

External-mount slaves leak from the boot end — fluid drips at the side of the gearbox. Concentric (internal-to-bellhousing) slaves leak internally — the leak is hidden inside the bellhousing and shows as low fluid level with no external leak. A failing concentric slave often means transmission removal for replacement, which is much more involved than an external-mount slave.

What to replace alongside

Always change the clutch fluid with new components. Inspect the flexible hose from master to slave (where fitted) and replace if it shows cracking. On cars with a long pedal-to-bellhousing pipe, inspect the pipe for corrosion. On internal-concentric slave cylinders, plan to replace the slave at the same time as the clutch — the access is the same.

Brand

LuK, Sachs, FTE, ATE supply OE-equivalent clutch hydraulics. The cylinder is application-specific.

Find clutch hydraulics for your car on the Clutch Hydraulics collection.