Clutch master cylinders form the first link between the engagement pedal and hydraulic clutch mechanism. Though these units are both simple and robust, even the best of them can fail over time through wear, weathering or abuse. Because there aren't many moving parts in a master cylinder, any failure is sure to be a fairly simple one. These failures can be the result of loose bolts, cracked fittings, or internal pressure leaks within the cylinder itself.
Hydraulic clutch systems are almost identical to hydraulic brake systems, and the symptoms of failure are generally similar. The first symptom of a failing master cylinder is a soft pedal, and there may be an audible hiss when engaging. The most telling symptom is incomplete disengagement, which can result in sudden and unwanted movement while idling at a stoplight.
Hydraulic clutch failures are almost without exception due to a lack of fluid pressure. Since a hydraulic clutch system uses pressure to disengage the transmission from the engine, any loss of system pressure will result in unintended engagement. These fluid leaks can be external, but are more often internal leakages caused by failing seals and system components.
The most common source of leaks on most vehicles is in the fittings that link the fluid line to master cylinder and clutch slave cylinder. Because single-cylinder hydraulic clutches are very sensitive to pressure loss, it doesn't take a huge leak to create a failure. A loss of as little as 0.1 psi will bleed enough system pressure out to sink a clutch pedal to the floor without disengaging the drive.
Internal Seal Leaks
Clutch master cylinders usually use a small piston about a 1/2-inch in diameter to pressurise the system. This piston's pressure seal resembles a washer, and can be made of plastic, rubber or other materials. This seal can harden, split or crack over time, allowing fluid to slip past the piston and into the low-pressure side. The master cylinder must be disassembled to replace this seal, a job that any competent mechanic should be able to accomplish in less than three hours.
Some of the other causes of master cylinder failure are a cracked piston spring, a bent pedal linkage, loose mounting bolts and fluid incompatibility. Some vehicles have displayed a tendency to overheat the clutch fluid if it is used excessively, but this is a fairly rare occurrence.