Because it costs very little, sits behind the gearbox, and fails often enough that reusing one is a poor bet against the labour of going back in.

It is the smallest and cheapest of the three main clutch components.

It is also under load whenever the pedal is down, which on urban work is a great deal of the time.

When it fails it makes noise first and can then seize, damaging the diaphragm fingers and sometimes the fork.

The labour to replace it alone is identical to the labour for a whole clutch, so the economics are entirely one-sided.

It comes in every kit for exactly this reason, and leaving the new one in the box to save a few minutes is a decision that returns.

By the pedal. A noise that appears when the pedal is pressed is the release bearing; one that disappears when the pedal is pressed is the input shaft bearing.

The release bearing only spins under load while the clutch is being held disengaged.

The gearbox input shaft bearing is loaded and turning while the clutch is engaged and the vehicle is in neutral, and unloads when the clutch is released.

So: noise in neutral with the pedal up, gone when pressed, points at the input shaft bearing.

Noise appearing as the pedal goes down, rising with pedal travel, points at the release bearing.

Both are inside the bellhousing, so the diagnosis changes what you order rather than how much work it is.

A combined release bearing and hydraulic slave cylinder mounted around the gearbox input shaft, inside the bellhousing.

It replaces the external slave cylinder and the release fork entirely, acting directly on the diaphragm fingers.

The advantages are a lighter, more consistent pedal and fewer linkage components to wear or go out of adjustment.

The disadvantage is that a hydraulic component with seals now sits in the least accessible place on the vehicle.

It must be replaced with every clutch on vehicles that use one - this is not a judgement call, and every reputable kit for such a vehicle includes it.

It also has to be bled, and many designs require a specific procedure or a vacuum bleeder because air trapped in the unit will not rise out.

Yes - the bearing slides along it, and a worn, dry or scored sleeve gives a heavy pedal and a bearing that runs crooked.

The guide sleeve is the tube on the gearbox front cover that the bearing carrier slides on.

Wear, corrosion or old hardened grease makes the bearing stick and release unevenly, which shows as a notchy or heavy pedal.

A bearing that binds on the sleeve also loads the diaphragm fingers unevenly and wears them into grooves.

Clean the sleeve and apply a thin film of the correct high-temperature grease - not so much that it can be thrown onto the friction faces.

A deeply scored or worn sleeve should be replaced, and on some gearboxes it is a separate part rather than integral to the cover.

Inspect them and replace if worn - both are cheap, both are inaccessible afterwards, and a worn pivot gives a heavy, vague pedal.

The fork wears where it contacts the bearing carrier and where it sits on its pivot.

Plastic pivot bushes and clips are common and become brittle with age and heat.

A fork that has worn a groove at its contact points changes the geometry, reducing the travel delivered to the bearing.

Retaining clips are the item most often lost or left off, and a fork that jumps off its pivot means the gearbox comes out again.

On vehicles with a concentric slave there is no fork at all, which removes this whole set of parts from consideration.

Riding the clutch, an incorrectly adjusted release mechanism, or a bearing that was reused - and the first is by far the commonest.

A driver resting a foot on the pedal keeps the bearing loaded and turning continuously at engine speed.

Holding the vehicle on a gradient with the clutch does the same and adds heat.

A misadjusted cable or hydraulic system that leaves the bearing in light contact all the time has the identical effect without the driver doing anything wrong.

Pedal free play exists precisely so the bearing sits clear when the pedal is up, and on designs that allow adjustment it must be set.

For fleets, early release bearing failures clustered on one driver or one vehicle are worth investigating as a behaviour or adjustment issue rather than a parts issue.

Nearly all modern release bearings are sealed for life and cannot be serviced; some heavy vehicle designs still provide a grease point.

A sealed bearing is filled at manufacture and replaced as a unit, which suits a component that is only reachable during a clutch job anyway.

Attempting to lubricate a sealed bearing achieves nothing and can force grease past the seal onto the friction faces.

Heavy commercial designs sometimes carry a remote grease point, and where one exists it belongs on the scheduled lubrication chart.

Missing that greasing point is a common cause of a heavy vehicle release bearing failing well before the disc.

Either way, the guide sleeve grease is separate and is applied at fitting on both types.