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Symptoms of a Bad Clutch Master Cylinder

By Mia Bevacqua, April 30, 2018

The clutch master cylinder is used to disengage the clutch on cars with manual transmissions. It’s connected to the clutch pedal and has a reservoir filled with hydraulic fluid — typically brake fluid.

The clutch lets you shift by engaging and disengaging the engine from the transmission. To understand how the master cylinder works, it’s helpful to have a basic knowledge of the entire clutch system. Or, you can jump ahead to the signs of a failing cylinder and get some repair advice.

The clutch system consists of the following components:

  • Flywheel: The flywheel provides a mounting point for the clutch on the back of the engine’s crankshaft — if the engine is turning, the flywheel is turning
  • Pressure plate: This is bolted directly to the back of the flywheel — so, if the engine is turning, the pressure plate is turning. The pressure plate squeezes the clutch disc hard against the flywheel.
  • Clutch disc: The clutch disc is covered in friction material and is placed between the flywheel and pressure plate. It’s attached to the transmission input shaft, so it only turns if the vehicle is moving and you’re not pressing the clutch pedal.
  • Release bearing: Also called the throwout bearing, this typically is a sealed or roller bearing. It rides on the transmission input shaft bearing retainer (which goes around the input shaft) and disengages the clutch disc when you press the clutch pedal. On some modern systems, the clutch slave cylinder is built in to the release bearing
  • Release mechanism: The release mechanism is a linkage between the clutch pedal and the release bearing. In a hydraulic system, this is the clutch master cylinder, slave cylinder and the lines that connect the two. 

The spring pressure of a pressure plate is very strong; some clutches have 10, 12 or even more springs in the pressure plate. Releasing this spring pressure takes a lot of force, which is why a pedal is used. Hydraulic systems make it easier to push the clutch in. 

When the clutch pedal is pressed, pressurized hydraulic fluid flows from the master cylinder to the slave cylinder. This causes the slave cylinder to act on the release bearing, disengaging the clutch. Most systems use a fork that pivots on a ball mounted to the transmission. The forked end goes around the release bearing, and the slave cylinder pushes on the other end.

Get it diagnosed by a professional
 

Signs of a failing clutch master cylinder

Clutch master cylinders are prone to leaks, and a faulty one can cause a number of problems. These are some of the most common.

Soft or loose clutch pedal: A clutch master cylinder that is leaking, internally or externally, can result in a soft clutch pedal or a pedal that has excessive free play. Free play is the distance the clutch pedal travels downward before the linkage or hydraulic system applies pressure against the pressure plate. Most cars will have a specification for the correct amount of free play. This measurement is usually between a half-inch and 1.5 inches. Erring on either side is bad for the clutch, and zero free play, or “riding the clutch,” will result in rapid wear on all clutch components.

Hard clutch pedal: A blocked compensating port or swollen seals inside the master cylinder can cause a hard clutch pedal. Another cause could be a worn transmission bearing retainer, which can make the pedal hard to push or cause it to jerk.

Shifting problems: To disengage the clutch, the release bearing must receive pressure from the master cylinder. A failed clutch master cylinder can result in a clutch that does not disengage, making the vehicle difficult or impossible to shift.

Fluid leak: A leaky clutch master cylinder will tend to leak into the insulation on the driver’s side floorboard and can only be seen by pulling back on the carpet below the clutch pedal. If you’re adding fluid but can’t find a leak, check here. 

» MORE: Get an estimate for your clutch master cylinder replacement

How to fix the problem

Before condemning a clutch master cylinder, the unit should be properly diagnosed. A technician will first check that clutch and transmission are OK.

If the master cylinder is found to be faulty, it should be replaced. This is a difficult job best left to a professional

The clutch hydraulic system will need to be bled of air after the master cylinder is replaced. Bleeding the air from a clutch can often be more challenging than bleeding brakes. Some systems will require special equipment to pressurize the bleeding process.

 

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