Diagnosing and Servicing Hydraulic Clutch Systems

As vehicle manufacturers began to embrace front- wheel drive, they started installing their engines transversely instead of longitudinally. This has some obvious benefits: easier, more compact packaging of the drivetrain, and no need to bend crank shaft rotation 90 degrees to get it to the wheels. However, when it comes to getting clutch pedal motion to the clutch release bearing, mechanical linkage would have to be quite a Rube Goldberg endeavor to make it work on a FWD. A cable would be a much simpler solution, so many 1980s vintage front-wheel-drive vehicles, and some rear wheel drive for that matter, used a cable between the clutch pedal and the release fork. Some were even self-adjusting. But a more elegant solution is the hydraulic clutch.

A hydraulic clutch "linkage" system is like a simple brake system. It has a master cylinder to provide hydraulic pressure, tubing and a flexible hose to carry the hydraulic fluid, and a slave cylinder which operates the release fork and release bearing.

The hydraulic system has several advantages over its mechanical and cable counterparts. The tubing can be easily routed wherever it needs to go. The pedal pressure is less due to the lack of friction that is inherent in any cable or mechanical system. And finally, the clutch pedal action is extremely smooth. The only down side to the hydraulic system is cost, which is why some vehicles made today still use a cable.

As you can see in Figure 1, the master cylinder is a simple single piston device. The reservoir supplies hydraulic fluid (usually brake fluid) to the cylinder inlet port. The reservoir may be mounted to the cylinder (as in the illustration) or mounted remotely and connected to the cylinder by a hose. The cylinder is bolted to the firewall and has a pushrod that that attaches to the clutch pedal. A hole in the cylinder body (inlet port) supplies fluid to the cylinder bore. The push rod moves the piston into the cylinder bore. As the piston primary cup passes the inlet port, it closes off the passage to the reservoir and pushes fluid out of the cylinder and into the tubing connecting the master cylinder to the slave cylinder. The secondary cup prevents any fluid from leaking out of the back of the master cylinder and into the passenger compartment.

The primary force returning the clutch pedal to its full upper position is the clutch pressure plate spring working against the slave cylinder piston, the fluid, and ultimately the master cylinder piston. The return spring inside the master cylinder helps push the piston back against the stopper ring to ensure that the inlet port is uncovered when the clutch pedal is released. The stopper ring prevents the piston from falling out of the back of the master cylinder.

As shown if Figure 2, the slave cylinder is also a single piston cylinder. As fluid enters the cylinder, it pushes the piston outward and causes the push rod to move the clutch release fork, ultimately moving the release (throw-out) bearing to disengage the clutch. As a typical push-style clutch disc wears thin, the release fingers of the pressure plate move toward the release bearing, reducing free play. This is why mechanical linkages require periodic adjustment.

As the clutch of a slave cylinder-equipped vehicle wears, the release fork pushes the slave cylinder piston further into the cylinder bore. While a few slave cylinder push rods are adjustable, most are not. Simply by making the cylinder longer than the normal pedal stroke required to release the clutch, the hydraulic clutch system is inherently self-adjusting. When the clutch is new, the piston is operating near the open end of the cylinder. As the clutch wears, the piston is pushed deeper into the cylinder, effectively "adjusting" the clutch to compensate for wear.

While most slave cylinders are external and are bolted to the outside of the bell housing, as shown in Figure 3, some slave cylinders are inside of the bell housing, as shown in Figure 4. This type of slave cylinder, also know as a concentric slave cylinder, mounts to the transmission bearing retainer and pushes directly on the release bearing. This design eliminates the release fork, but if the cylinder needs to be replaced, the transmission obviously needs to be removed.

The tubing used to connect the master cylinder to the slave cylinder is often brake line-type metal tubing. If metal tubing is used, there will be a flexible hose between the tubing where the transition is made from the chassis to the engine. This allows for movement of the engine without damaging the tubing. But some vehicles use plastic-like tubing. This tubing can be easily damaged, so be careful when working with it. Plastic tubing does not use flare nut fittings to attach it to the cylinders. They use air conditioning-like fittings that require as special tool to remove.

Failures of the hydraulic clutch systems usually result in fluid leakage. The typical scenario is a customer complaint that he can’t shift the transmission without gear clash, or possibly can’t shift at all. As with a brake system, as long as there is sufficient fluid in the system, everything works fine.

Fluid loss from a leaking master cylinder, slave cylinder, or hose, causes the fluid level in the reservoir to go down. Then comes that one dreaded pedal stroke where there is no fluid in the reservoir and the master cylinder swallows a gulp of air instead of fluid. Now we have compressible air in the hydraulic system. The result is a mushy clutch pedal feel and reduced slave cylinder piston travel as the air in the hydraulic system is compressed. Since the slave cylinder piston will no longer travel its full stroke, the clutch does not completely disengage, resulting in difficult or impossible shifting.

External slave cylinder leakage is easily diagnosed, as hydraulic fluid will be leaking from the dust boot area of the cylinder. Occasionally you may find that the boot is full of fluid, but not yet dripping. Squeezing the boot will cause fluid to be expelled from it. There should never be any fluid in the boot. If squeezing the boot gets your fingers wet with brake fluid, you’ve found a defective slave cylinder.

The internal, or concentric, cylinder may be more difficult to diagnose. Clutch master cylinder reservoirs tend to be small and therefore hold a small volume of fluid. A leaking internal cylinder will drip inside the bell housing where there is often a considerable amount of clutch dust that absorbs the fluid. If you see evidence of hydraulic (brake) fluid inside the bell housing, or dripping from it, you have a defective cylinder.

If you find that the reservoir level is low, but see no signs of leakage at the bell housing, it does not mean that the cylinder is not leaking. The best way to condemn the cylinder in this case is by a process of elimination. Carefully inspect the entire system for leakage.

Check the master cylinder, lines and hose. If no other leaks are seen, the fluid must be leaking from the internal slave cylinder and the clutch dust is absorbing the fluid. If the customer or someone else has topped of the master cylinder several times because of a low fluid level and the slave cylinder is leaking, that should be a sufficient volume of fluid to saturate the clutch dust and create a visible leak in the bell housing.

Clutch master cylinder leakage, like a brake master cylinder, can be internal or external. External leaks show up as fluid dripping from the pushrod area inside the car. The pushrod will be wet, and there may even be fluid on the carpet or floorboard insulation. This external leakage would be due to a worn secondary cup on the piston of the master cylinder.

While kits are available to rebuild some master cylinders, the sure fix is a new unit. Pitting of the cylinder bore is often the cause of cup or seal damage. While cast iron cylinders can be lightly honed, aluminum cylinders cannot. Any pitting of the bore is a comeback waiting to happen.

The scenario for internal leakage is this: a driver is stopped, such as at a traffic light, with the vehicle in gear and her foot depressing the clutch. Even though the clutch pedal is held to the floor, the clutch begins to engage which either makes the vehicle creep forward or stalls the engine. This happens when the hydraulic pressure in the system pushes back past the primary cup in the master cylinder. The fluid pushes back into the reservoir since the secondary cup prevents it from leaking out the back of the master cylinder. The slave cylinder piston retracts and the clutch begins to engage. If you find this symptom with no external leakage, you have a worn primary cup requiring master cylinder repair or replacement.

Whenever there is a loss of hydraulic pressure while the clutch is disengaged, whether the leak is internal or external, the clutch pedal may not fully return. In fact, if severe enough, the pedal may stay at the floorboard. Failure of the clutch pedal to return can happen any time there is a loss of hydraulic pressure while the clutch is disengaged, such as might happen if there is a severe external fluid leak.

Air, of course, is not compressible. And, just like in a brake system, air in the clutch hydraulic system can cause a mushy pedal and incomplete clutch disengagement. This can cause hard shifting or gear clash when shifting, especially into reverse, since most reverse gears are not synchronized.

Air in the system is often the result of low reservoir fluid level due to an external leak. The clutch will work normally as the reservoir depletes until that one unfortunate pedal stroke when the reservoir finally goes dry and the master cylinder takes a gulp of air. Occasionally air will enter the system during the clutch pedal up stroke with no loss of fluid. This can occur when there is a slight negative pressure within the system, and a worn piston seal allows air around it. If you bleed air from a system and the air returns within a week or two, suspect air leaking into the system.

Component replacement is generally straightforward. Always follow the manufacturer’s procedures for removal, installation and bleeding. Don’t forget to torque all fasteners to spec. Some master cylinder push rods are adjustable for length. This is to allow for complete return of the piston to the rear of the cylinder and provide proper pedal stroke length.

There should always be as small amount of freeplay between the master cylinder push rod and the piston. Some clutch pedals have an adjustable stop at the top of the pedal travel. These adjustments can affect the stroke of the clutch pedal, and therefore the piston and release bearing travel. Too much travel may damage the pressure plate, and too little may cause the clutch to drag or engage very low to the floor. If the vehicle has these adjustments, don’t overlook them, especially if replacing the master cylinder.

Although brake fluid is used almost universally for clutch hydraulic systems, there may be the odd duck out there that requires something else. The reservoir cap is often stamped with the type of fluid required.

Fluid contamination often happens when a DIYer decides to top off the clutch reservoir with something other than the recommended fluid. In my experience, the incorrect fluid of choice is gear oil. I guess the thought process is: clutch plus manual transmission equals gear oil! Needless to say, after this bit of genius, every rubber component in the system must be replaced. Master cylinder seals, slave cylinder seals (or assemblies), and hose must go. All metal lines must be flushed.

Some manufacturers, notably Saturn, do not service any individual component of the clutch hydraulic system. The master cylinder, slave cylinder and tubing are sold as an assembly. It even comes filled with fluid and bled of air. If nothing else, this policy would ensure that you fix the hydraulic system no matter what the problem.

Some systems, particularly Ford and GM trucks and others with an internal slave cylinder, can be particularly difficult to bleed. The "book" procedure for bleeding these systems is rather arduous, and often does not achieve the desired result in a reasonable amount of time.

We have found that "reverse" bleeding these systems works quickly every time. This is done using a fluid injecting tool that is often used for bleeding brakes. A hose attaches the tool to the slave cylinder bleeder screw and fluid is pumped backwards through the system. Since the air naturally wants to go toward the top of the system, this method effectively pushes the fluid and air up into the master cylinder and into the reservoir.

While hydraulic clutch systems are not high-tech, you can still find an occasional TSB on them. Ford trucks from 1993 to 1997 had some issues with internal slave cylinders causing hard clutch pedal operation. Some 1984-1990 Ford trucks also had problems with cracking of the firewall where the master cylinder mounts. A kit is available to reinforce the firewall.