When a clutch pedal goes soft, sinks to the floor or stops working altogether, the problem is often not the clutch itself but the hydraulics that operate it. Most modern manual cars use a master cylinder and a slave cylinder to do this job, and either one can fail.
They cause similar symptoms, so it can be hard to tell which is to blame. Here's how they work, the signs of each one failing, and how to narrow it down.
How the clutch hydraulics work
When you press the clutch pedal, it pushes a piston inside the master cylinder, which sits on the bulkhead behind the pedal box. That pushes fluid down a pipe to the slave cylinder at the gearbox. The slave cylinder then moves the clutch release fork or bearing, which disengages the clutch so you can change gear.
On many cars, the fluid comes from the same reservoir as the brakes, so it's usually brake fluid.
There are two main types of slave cylinder. An external slave sits on the outside of the gearbox and pushes a fork. A concentric slave cylinder sits inside the bell housing, around the gearbox input shaft, and doubles as the release bearing. Concentric slaves are common on modern cars and are much harder to reach.
Symptoms of a failing master cylinder
- The pedal slowly sinks to the floor while you hold it down at a standstill
- The pedal feels soft or spongy, and bleeding doesn't fix it for long
- The pedal sticks down or returns slowly
- Brake fluid on the carpet or under the dashboard near the pedals
- Gear changes get harder as the car warms up
A slowly sinking pedal with no visible leak is a classic sign. The seals inside the master cylinder are letting fluid bypass the piston, so pressure drops away even though no fluid is escaping.
Symptoms of a failing slave cylinder
- A soft pedal and a falling fluid level in the reservoir
- Fluid leaking from the outside of the gearbox, or dripping from the bottom of the bell housing
- Difficulty selecting gears, especially first and reverse
- The clutch not disengaging fully, so the car creeps with the pedal down
- The pedal staying on the floor after being pressed
With an external slave, a leak is usually easy to spot as a wet patch around the cylinder. With a concentric slave, the leak is hidden inside the bell housing, so you'll often see fluid dripping from the joint between the engine and gearbox instead.
How to tell which one it is
Start with the fluid level. If it keeps dropping, there's a leak, and you'll need to find where.
Check the footwell. Pull back the carpet under the pedals and feel for damp patches. Brake fluid there points to the master cylinder.
Check the gearbox. Look around the slave cylinder or at the bottom of the bell housing. Fluid there points to the slave.
No leak, but a sinking pedal? That points to internal wear in the master cylinder.
If there's no leak and the pedal is just spongy, it may simply be air in the system. Try bleeding it first. Our guide to bleeding a clutch at home walks through it. If the pedal is fine after bleeding but goes soft again within days, one of the cylinders is likely failing.
Should you replace both?
It depends on the car and the age of the parts. Many mechanics replace both at once on older cars, because if one has failed from age, the other is often not far behind. On cars with a concentric slave cylinder, it's common to replace the slave whenever the clutch is replaced, because reaching it means taking the gearbox off anyway.
If the clutch itself is also worn, it makes sense to combine the jobs. Our guide to clutch replacement costs explains how the labour adds up.
Replacing them
An external slave cylinder is usually a straightforward job: a couple of bolts, the fluid pipe, then bleed the system. A master cylinder is usually held on by two nuts behind the pedal box, but access can be tight under the dashboard. A concentric slave means removing the gearbox, which is a garage job for most people.
Whichever you replace, use fresh fluid of the right type and bleed the system thoroughly afterwards.
We stock a wide range of clutch master cylinders and slave cylinders, along with other clutch parts. These parts are specific to the model, so match the part number on your old one, or send us your VIN and we'll help you find the right one.
This article is a general guide only. If you're not confident working on hydraulic systems, ask a qualified mechanic.