The clutch operation with the external slave cylinder is really called a semi-hydraulic, whereby 'semi' comes from the Latin and means 'half'. What is meant, is only that the hydraulics don't reach to the pressure plate. There is still the shifter lever.
Double-clutches are still semi-hydraulically applied ...
We will however, now concentrate on the (full-) hydraulics without a lever mechanism. The respective pipe, coming from the master cylinder, leads us to the clutch housing and ends in a ring cylinder. This component is called the central shifter bearing.
In the past, when changing the clutch, the shifter bearing was replaced as well, but not necessarily the slave cylinder because it was easily accessible, if it wasn't in the clutch housing. Nowadays, with the shifter lever, the central shifter bearing is also changed, because after being assembled again it is no longer accessible and because, together with the release lever, it forms a unit.
It goes on even further. The so-called cover-fixed central shifter bearing does not rest on the gearbox casing, but remains fixed to the clutch cover, thus taking part in the axial oscillations. Were it fixed to the clutch casing, these oscillations would influence it's shifting behaviour or be transferred to the clutch pedal.
At this point, we don't want to question, where the pressure for the shifting comes from. Apart from the usual clutch pedal, there could also be a type of automatic clutch or an automated manual gearbox. The latter of course, needs sensors which are either integrated into the clutch cylinders or are mounted on the side. Exactly this, the sensitive letting in of the clutch, without letting it slip for too long, perhaps made more difficult by having a heavy load, is one of the most difficult tasks, even with modern electronics.
The self-adjustment functions basically the same as with the semi-hydraulics. The casing of the central shifter bearing is more than likelyto be made of very heat resistant synthetic material, than of Aluminium. In those shifters which are not fixed to the cover, the casing may form a unit with the guide-pipe. The use of die-cast synthetics as opposed to Aluminium is of course, also a question of economics.
Some central shifter bearings carry a red-marked assembly safeguard. Under no circumstances should this this be loosened, before the assembly is completed. Due to the leaks which are then to be expected, this should never be re-used, if one has loosened the safeguard too early. Whatever the case may be, the accompanying assembly instructions must be adhered to. 08/13