Basic Hydraulic Clutch
Most modern cars use a hydraulic clutch. When you put your foot on the clutch pedal you are using mechanical leverage to depress a piston. The piston pushes fluid through a tube which pushes another piston to depress the clutch mechanism.
The hydraulic clutch assembly is filled with clutch fluid. The fluid acts as a connector between the top piston and the bottom piston. Because the fluid can not be compressed there should be no loss of force between the Top piston compression and the bottom piston compression, even though the hydraulic fluid tubing is very thin.
1. Fluid reservoir tank
2. Top Clutch Piston(Master Cylinder)
3. Clutch Pedal
4. Clutch actuator
5. Bottom Clutch Piston(Slave Cylinder)
Power Assist Clutch
A hydraulic clutch may also have a power assist or booster attached to the hydraulic system. This is usually in the form of a vacuum pump to assist you in pushing the clutch pedal in. Often this vacuum assist will also be linked to the braking system in the car to assist with stopping the car with minimal effort.
The car's power assist can be felt when the car's engine is not turned on. The clutch and brake will be much harder to apply.
Over time the pistons in the master cylinder and slave cylinder can wear to a point that they will leak. When they leak air is introduced into the hydraulic lines. Air, unlike fluid can be compressed. If there is air in the clutch line the clutch slave cylinder will not fully depress as the compressed air will result in less travel of the fluid.
Air in the clutch line will eventually make changing gears harder. You might find it hard to get the car into gear or hear gear grinding between shifts. Oil might also be visible leaking out of the master or slave cylinders and the hydraulic fluid in the reservoir will decrease.
After the clutch has had either the master cylinder or slave cylinder replaced and new hydraulic fluid has been put into the lines the clutch has to be bled. Bleeding expels all the air out of the line leaving only clutch fluid. To bleed the clutch a valve is opened near the slave cylinder and fluid is pumped through the lines. The open valve allows fluid and air to flow out, but stops any air from flowing back into the hydraulic line. A clutch bleeding kit is available to assist with clutch bleeding.