A hydraulic fault gives a clutch that will not release - hard gear selection from rest and a graunch into reverse. A worn clutch slips instead.

Slip is the clutch failing to hold: engine speed rises without acceleration, worst in a high gear under load.

Drag is the clutch failing to let go: the vehicle is still being driven with the pedal on the floor.

Nearly all drag complaints on a hydraulic system are hydraulic - air, a failing master, or a failing slave - rather than mechanical.

A quick test is to press the pedal several times in quick succession. If the release improves, there is air or a failing seal.

Checking this before removing a gearbox is the highest-value few minutes in clutch diagnosis, and it is skipped often enough to be worth stating.

Loss of hydraulic pressure - a failed seal, a leak, or air - and on some systems the pedal has to be lifted by hand because there is no return spring.

A sudden total loss usually means a burst pipe, a blown seal or a slave cylinder that has come apart.

A gradual loss over days or weeks is a seal passing internally, with the fluid level falling slowly or not at all.

Check the reservoir first. Many clutch systems share the brake reservoir, so a clutch leak shows as a brake fluid warning.

Look at the slave cylinder and the pipe for external wetness, then look at the pedal box for fluid behind the master cylinder.

A pedal that stays down with no leak anywhere and a full reservoir is usually air or a master cylinder failing internally.

In the manufacturer's way - and many need vacuum or pressure bleeding, or a specific sequence, because air will not rise out of a concentric slave.

Conventional pedal-and-bleed-nipple bleeding works on many systems with an external slave and a well-routed pipe.

Where the pipe rises and falls between master and slave, air is trapped at the high point and no amount of pedal pumping moves it.

A concentric slave inside the bellhousing is the worst case, and pressure or vacuum bleeding is frequently the only method that works.

Reverse bleeding - pushing fluid from the slave upwards to the reservoir - is effective because air naturally travels that way.

Keep the reservoir full throughout. On a shared reservoir, draining it also puts air into the brakes, which turns one job into two.

Yes, on the same schedule as the brake fluid it usually shares a reservoir with - and it is routinely forgotten.

Conventional brake fluid absorbs moisture from the air, and the clutch circuit is part of the same system on most vehicles.

Water in the fluid corrodes cylinder bores from the inside, which is the underlying cause of most cylinder failures.

The clutch slave sits near the gearbox and often near the exhaust, so its fluid runs hotter than the brake circuit's and degrades faster.

A brake fluid change that does not include bleeding the clutch circuit leaves the oldest, wettest fluid exactly where it does most harm.

Make the clutch circuit part of the brake fluid service on the job card, or it will be missed every time.

On vehicles fitted with one, yes - it is inaccessible without removing the gearbox, so the decision is made by access rather than by condition.

The unit sits around the input shaft inside the bellhousing and carries the release bearing.

Replacing it later costs the entire clutch job's labour for a modestly priced part.

Every reputable four-piece kit for such a vehicle includes it, and fitting a three-piece kit instead is a false economy that shows up within a year or two.

Check the specific bleeding requirement before starting, because some units must be bled before the gearbox is finally seated.

Handle it carefully during fitting - the seal lip is easily damaged, and a damaged seal means the gearbox comes straight back out.

Air in the system, a swelling flexible hose, or fluid that has boiled near a heat source.

Air compresses and fluid does not, so any sponginess means gas somewhere in the circuit.

A flexible section that has softened with age expands under pressure instead of transmitting it, giving the same feel with no air present.

Fluid that has been overheated near the exhaust or gearbox can vaporise, which produces a pedal that is fine cold and poor when hot.

Heat shielding and correct pipe routing are the cure for that, not repeated bleeding.

If bleeding cures it temporarily and it returns, a seal is drawing air in on the return stroke - that is a cylinder replacement, not a bleeding problem.

Only where the manufacturer offered both, because the pedal ratio, mountings and release travel are designed around one system.

A cable clutch is simple, cheap and adjustable, and it transmits engine movement and friction into the pedal.

A hydraulic system gives a lighter, more consistent pedal and needs no adjustment as the disc wears.

The pedal box, the bulkhead mounting and the release lever geometry differ between them, so conversion is a fabrication exercise rather than a parts swap.

Where a manufacturer offered both on the same platform, the parts exist and the conversion is documented - that is the only straightforward case.

For a fleet the more useful question is keeping the system that is fitted properly adjusted and its fluid fresh, which prevents most of the complaints either type generates.