Hydraulic clutch control systems are fairly simple. They consist of a master cylinder that is connected to the clutch pedal via a pushrod, and a slave cylinder that acts on the clutch via a linkage or control arm, commonly known as a clutch fork since it resembles a fork with two tines.
When the clutch pedal is depressed, a moving piston in the master cylinder displaces a quantity of brake fluid into the slave cylinder. The displaced fluid pushes on a moving piston in the slave cylinder, which then acts on the clutch fork, which in turn, acts on the clutch release bearing to disengage the clutch. The opposite happens when the clutch pedal is released; the fluid returns to the master cylinder reservoir as a result of the pressure plate diaphragm pushing against the release bearing, which action reverses the direction of flow in the hydraulic system when pressure is removed from the clutch pedal.
Although hydraulic clutch control systems are fairly simple, these systems come with their own problems that have the potential to stop the clutch from working. We take a look at the common issues below.
Since most hydraulic clutch control systems work with brake fluid that is hygroscopic (absorbs moisture from the atmosphere), the concentration of water in the brake fluid can reach levels that are high enough to cause corrosion in both the master and slave cylinders. Over time, the corrosion can cause the sliding pistons in the cylinders to stick or bind, which in turn, can cause the clutch pedal not to return to its rest position when pressure is removed from the clutch pedal.
The easiest way to prevent this from happening is to replace the brake fluid in the control system every two years or so, or more frequently in humid climates.
Although the hydraulic clutch control systems in most vehicles require brake fluid that conforms to the DOT 4 standard, brake fluid that conforms to the earlier DOT 3 standard can be used in hydraulic clutch control systems that require DOT 4 brake fluid. Be aware however, that brake fluid that conforms to the DOT 5 standard must NOT be mixed with either DOT 3 or DOT 4 brake fluid. DOT 5 brake fluid has a silicone base, which is NOT compatible or miscible with ANY other types of brake fluid, meaning that mixing the two types of brake fluid can result in unexpected failure of the clutch control system.
The sliding pistons in both master and slave cylinders are fitted with neoprene rubber seals that contain the working pressure in the system. However, for these seals to be effective, the bores of the cylinders have to be free of corrosion, pitting, or other irregularities that can cause brake fluid to leak past the seals when the system is under pressure.
While regular replacements of the brake fluid in the system can prevent fluid leaks caused by corrosion, the small rubber seals in the system wear out over time, which usually results in fluid leaks. It should be noted though that regardless of the cause of a fluid leak in a clutch control system, excessive fluid loss could cause air to enter the system, which in turn, can prevent normal operation of the clutch.
If you are experiencing any symptoms, contact your local Repco Authorised Service Centre for advice.