Hydraulic Clutch System Failure Prevention

Most clutch actuation systems are semi-hydraulic, when you push down the clutch pedal, a moving piston in the master cylinder pushes hydraulic fluid into the slave cylinder. The displaced fluid pushes on a moving piston in the slave cylinder, which acts on the clutch fork, this then pushes or pulls the clutch release bearing on the cover diaphragm, causing the clutch to disengage.

What Can Hydraulic Clutch Systems Fail To Do?

Although hydraulic clutch control systems are relatively simple, they do have their own set of issues that can cause the clutch to stop working. 

The most common problem with clutch master and slave cylinders is hydraulic fluid contamination or the use of the incorrect type of hydraulic fluid. A single drop of contaminants, such as water, air, dust, or metal particles, can cause the rubber seals within the cylinder to swell over time. When this occurs, the cylinder can no longer function properly and may seize. Contaminants can be introduced in different ways including high humidity, and wear on moving components. Another cause of failure is when replacing CSCs is not following the correct bleeding procedure, so the installer must take special care to follow it.

The clutch pedal is not returning

Because most hydraulic clutch control systems use hygroscopic (absorbs moisture from the atmosphere) brake fluid, the concentration of water in the brake fluid can reach levels high enough to cause corrosion in both the master and slave cylinders. Corrosion can cause the sliding pistons in the cylinders to stick or bind over time, preventing the clutch pedal from returning to its rest position when pressure is removed from the clutch pedal.

The simplest way to avoid this is to replace the hydraulic fluid in the control system every two years, or more frequently in humid climates.

Although the hydraulic clutch control systems in most vehicles require hydraulic fluid that conforms to the DOT 4 standard, fluid that conforms to the earlier DOT 3 standard can be used in hydraulic clutch control systems that require DOT 4 fluid. Be aware however, that hydraulic fluid that conforms to the DOT 5 standard must NOT be mixed with either DOT 3 or DOT 4. DOT 5 fluid is silicone based, which is not compatible any other types of hydraulic fluid, meaning that mixing the two types of brake fluid can result in unexpected failure of the clutch control system.   

Leak of clutch fluid

The sliding pistons in both the master and slave cylinders are equipped with neoprene rubber seals that keep the system’s working pressure in check. However, in order for these seals to be effective, the cylinder bores must be free of corrosion, pitting, or other irregularities that could allow brake fluid to leak past the seals when the system is under pressure.

While replacing the clutch fluid in the system on a regular basis can help to prevent leaks caused by corrosion, the system’s small rubber seals wear out over time, resulting in leaks. It should be noted, however, that regardless of the cause of a fluid leak in a clutch control system, excessive fluid loss can cause air to enter the system, preventing normal clutch operation.