If your clutch pedal feels spongy, sinks to the floor or doesn't spring back properly, there's a good chance there's air in the hydraulic system. Bleeding it pushes the air out and usually brings the pedal back to normal.
It's a job many people can do at home with a few basic tools and a helper. This guide covers what you need, the usual method, and what it means if bleeding doesn't fix the problem.
How a hydraulic clutch works
Most modern manual cars use a hydraulic clutch. When you press the pedal, it pushes a piston in the master cylinder, near the pedal box. That forces fluid down a pipe to the slave cylinder at the gearbox, which moves the clutch fork or release bearing and disengages the clutch.
Fluid doesn't compress, so the pedal feels firm. Air does compress, so even a small bubble in the system makes the pedal feel soft and can stop the clutch releasing fully. That can make it hard to get into gear, especially first and reverse.
Air usually gets in when the system has been opened up, for example after replacing the slave cylinder, master cylinder or a hose, or when the fluid level has dropped too low.
What you'll need
Fresh hydraulic fluid of the right type. On most cars, the clutch shares its reservoir and fluid with the brakes, so it's usually DOT 4 brake fluid. Check your handbook, because a few cars use something different, and mixing the wrong fluid can damage the seals. Always use a fresh, sealed bottle. Brake fluid absorbs moisture from the air once it's opened.
A ring spanner that fits the bleed nipple, usually 8mm, 10mm or 11mm.
A length of clear tube that fits snugly over the nipple, and a jar or bottle to catch the old fluid.
A helper to press the pedal. If you're on your own, a one-person bleed kit or a vacuum bleeder will do the job.
Rags, gloves and eye protection. Brake fluid strips paint and isn't something you want in your eyes, so wipe up any spills straight away and keep a cloth over painted surfaces under the reservoir.
Find the bleed nipple
The bleed nipple is on the slave cylinder or on the pipe running to it. On many cars the slave cylinder sits on the outside of the gearbox, and the nipple is easy to reach from above or underneath.
Some cars use a concentric slave cylinder, which sits inside the bell housing around the gearbox input shaft. In that case, the bleed nipple is normally on a short pipe or bracket on the outside of the gearbox. It can be awkward to reach, and you may need to remove the battery, air box or an undertray to get to it.
If the car needs to be raised, use proper axle stands on solid ground. Never work under a car held up only by a jack.
The usual method: pedal bleeding
This is the two-person method most mechanics learn first.
Top up the reservoir to the MAX line. Keep an eye on it the whole time. If it runs dry, you'll draw more air in and have to start again.
Push the clear tube onto the bleed nipple and put the other end in your jar, with a little fresh fluid in the bottom so the tube end stays submerged.
Ask your helper to press the clutch pedal slowly to the floor a few times, then hold it down.
With the pedal held down, open the bleed nipple about a quarter to half a turn. Fluid and air will push out into the tube. Close the nipple before your helper lets the pedal back up.
Repeat until the fluid coming through the tube is clear and has no bubbles at all. Top up the reservoir every few strokes.
Close the nipple firmly, but don't overtighten it. They're small and easy to shear off.
Check the pedal. It should feel firm and return to the top on its own, and the clutch should bite at a sensible point.
When pedal bleeding doesn't work
Some clutch systems are stubborn. The pedal travel is short, and air can get trapped at a high point in the pipe or inside the master cylinder. If you've bled it a dozen times and the pedal still feels soft, try one of these.
Gravity bleeding. Fill the reservoir, open the bleed nipple and let fluid drip through slowly on its own, topping up as you go. It can take a while but often shifts small bubbles that pedal bleeding misses.
Vacuum bleeding. A hand pump or compressed air vacuum tool on the nipple pulls fluid through the system. Good if you're working alone.
Reverse bleeding. Fluid is pushed up from the bleed nipple to the reservoir using a large syringe. Because air rises, this can work well on systems where the master cylinder sits higher than everything else. You'll need to draw some fluid out of the reservoir first so it doesn't overflow.
Flicking the pedal. On some cars, pushing the pedal down and then letting it snap back up quickly a few times helps work trapped air back up into the reservoir.
If the pedal still goes soft
If the pedal is firm after bleeding but goes soft again after a day or two, or the fluid level keeps dropping, the system is leaking somewhere.
Check around the slave cylinder for fluid. On an external slave, you may see wet patches on the gearbox casing or drips underneath. On a concentric slave, the leak is inside the bell housing, so you might see fluid dripping from the bottom of the gearbox where it joins the engine.
Look at the master cylinder too. A leaking master cylinder often leaves fluid on the carpet behind the pedals.
A pedal that slowly sinks to the floor while you hold it down, with no visible leak, usually means the seals inside the master cylinder are failing. Bleeding won't fix that, and the cylinder needs replacing.
We stock clutch slave cylinders, master cylinders and other clutch parts for most popular cars. 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.
Before you drive
With the engine off, press the pedal a few times and check it feels the same each time. Start the car with the clutch down, select first and reverse, and make sure they go in cleanly. Check the brakes too if the system shares a reservoir, and make sure the fluid level is between MIN and MAX.
Old brake fluid is hazardous waste. Take it to your local recycling centre rather than pouring it away.
This article is a general guide only. Every vehicle is different, so follow the manufacturer's procedure where one exists, and if you're not confident working on hydraulic systems, ask a qualified mechanic.