The clutch operatation can be carried out mechanically (mostly by cable) or hydraulically. The hydraulics transfer the force for the disengaging of the clutch from the pedal to the release bearing. This transferance becomes smoother, the lower the force lost through friction is. This way, greater distances between the pedal and the clutch (e.g., in the case of buses) are possible.
The hydraulics with the master cylinder (in the picture on the right) and the slave cylinder (on the left) take the place of the Bowden cable. The brake fluid is taken from the reservoir (mounted high enough) of the brake system or from a separate container. This pipe leads to the master cylinder (in the picture, at the top). The amount of fluid sinks as the brake-pads wear down, therefore, in this situation the container should not be filled to the brim.
The pressure is distributed from the master cylinder and stays the same for the respective pedal pressure everywhere. By using various piston diameters, a manufacturer can set a transformation ratio, e.g., by using a smaller master cylinder diameter, the pedal force can be reduced. The spring in the slave cylinder presses the piston through the linkage, even when no activity is taking place, against the release bearing, which then lies on the tongue of the membrane spring and always rotates with it.
Earlier, only the hydraulic clutch operation was self-adjusting, nowadays it is also the standard in cable operated systems.
Testing for leakage: (only for testing purposes!) Keep the engine running for very long time, holding the clutch depressed and the 1st gear engaged. If at any point, the vehicle starts to move forwards by itself, then there is a leak in the hydraulics.
function of the clutch is of course, something quite different. In this case the handbrake must be activated, then an attempt to pull-off should be made. If the clutch is let in, with a gear engaged, the RPM must drop rapidly or the engine should stall, if neither happens then the clutch is slipping. 09/11