In a hydraulic clutch, the clutch pedal pushes the master cylinder piston. This forces hydraulic fluid out of the cylinder, moving the slave cylinder piston. Short of a cut hydraulic line, there are three common causes for a clutch master cylinder failure: seal failure, pitted cylinder walls and air in the system.
Pitted Cylinder Walls
Pitted cylinder walls are the result of grit getting into either or both the mast and slave cylinders, though grit in the master is far more common than in the slave. Badly pitted cylinder walls can prevent the master from fully returning to the rest position. This leaves the cylinder partially engaged and the clutch intermediate between engaged and disengaged. Replacing or rebuilding is required.
A front seal failure allows hydraulic fluid to seep back into the master cylinder reservoir rather than going through the outlet pipe, resulting in decreased or no movement of the slave piston and resultant failure to fully disengage the clutch from the transmission. The remedy is to replace-rebuild the master cylinder.
A rear seal failure allows fluid to leak out of the rear of the master cylinder. You will know you have a rear seal failure if there is hydraulic fluid on the clutch pedal. The solution is the same as for a front seal leak.
Air in the System
When air bubbles get into the hydraulic fluid line, problems occur. The compressed air can cause either or both the master and slave cylinders to fail by preventing them from fully depressing or fully releasing. Luckily, you only need to bleed the system and carefully top it back up with new fluid to correct this problem.