Though many late-model automatic transmissions now have a manual shift mode, manual gearboxes are still the transmission of choice for many automotive enthusiasts. Even so, the system is not trouble free.
Of course, accurate diagnosis is the key to replacing the correct part. If leaks are found, the component must be replaced. Low fluid in the master clutch cylinder is usually a clue that something is leaking. The master cylinder, tubing and slave cylinder should be visually inspected for leaks. If the vehicle has an internal hydraulic throw-out bearing, it may not be possible to inspect it without removing the transmission. The next best thing would be to look for fluid leaks along the bottom of the bellhousing.
If no leaks are found and the master cylinder reservoir is full, failure of the clutch to disengage may be the result of a bypassing piston seal in either the master or slave cylinder or, in some cases, by a mechanical problem with the clutch itself.
If the vehicle has a slave cylinder on the bellhousing and the release fork moves when the clutch pedal is depressed, the problem is in the clutch itself. The transmission will have to be removed to inspect the clutch. No movement of the slave cylinder or release fork when the clutch pedal is depressed indicates a failure to hold pressure in the hydraulic system.
A brake pressure gauge can also be used to troubleshoot the system. To use this tool, the line to the master cylinder to the slave cylinder or hydraulic throw-out bearing should be connected to the gauge. If the pressure reading on the gauge is normal and holds steady when the clutch pedal is depressed, the problem is not the master cylinder but the slave cylinder or hydraulic throw-out bearing. If the gauge shows lower than normal pressure, or the reading drops as pedal pressure is maintained, the master cylinder piston seal is leaking and the master cylinder needs to be replaced.
Though it may be possible to rebuild some master or slave cylinders with a kit, ones with plastic or aluminum cylinders must be replaced. On some vehicles, the master cylinder, slave cylinder and tubing are all connected and must be replaced as an assembly and not as individual parts.
If the fluid in the system is badly discolored and contains a lot of rust or sediment, the whole system should be replaced. Also, if the fluid has been accidentally contaminated with any liquid other than the recommended hydraulic fluid, the master and slave cylinders may have to be replaced because oil can cause the seals to swell excessively.
If a customer needs a master cylinder, slave cylinder or hydraulic throw-out bearing, you should recommend replacing all of the components at the same time. Replacing the whole system will restore the system to like-new condition and save the vehicle owner the inconvenience and cost of having to fix the clutch linkage again in the near future.
Customers who are replacing a clutch should also be advised to carefully inspect the hydraulic clutch linkage for leaks and check the fluid for signs of corrosion or contamination. If any problems are found, the system should be replaced.
It may be a good idea to replace the slave cylinder anyway when changing a clutch. Here’s why: Replacing the clutch changes the operating height of the clutch release fingers or diaphragm. This, in turn, changes the rest position and travel of the piston inside the slave cylinder. This may cause the seal to rub over a corroded area of the bore, or to interfere with rust or dirt that has built up behind the seal. Either way, the seal wears and eventually leaks. It’s like telling a customer he should replace his calipers when doing a brake job. It may not be absolutely necessary at the moment, but it can prevent problems down the road.
Also, if a customer is replacing a conventional throw-out bearing, the hydraulic linkage should be inspected to make sure the slave cylinder isn’t leaking.
HYDRAULIC LINKAGE PROBLEMS
The most common problem with a hydraulic clutch linkage is that the clutch won’t release. The underlying cause may be a worn or leaky piston seal in the master or slave cylinder, loss of hydraulic fluid from the system (leaks) or air in the system.
Like brake fluid, hydraulic fluid is incompressible. So to operate the clutch properly, there can’t be any air in the master or slave cylinders, or the line that connects these two components. Air in the system will make the pedal feel soft and spongy instead of firm. Bleeding the system will get rid of the air, but if the piston seals are worn or there’s a leak somewhere, the system won’t hold pressure and air will get back inside.
On most vehicles, the slave cylinder has a bleeder screw so the system can be bled to remove air. The bleeding procedure is a two-person job, unless a pressure bleeder is used or a one-way bleeder adapter is attached to the bleeder screw. To bleed the system manually, the bleeder screw on the slave cylinder is opened. Then the clutch pedal is slowly depressed to push fluid from the master cylinder to the slave cylinder. The bleeder must then be closed while the clutch pedal is released, otherwise air may be sucked back into the system. This process is repeated as many times as needed until there are no air bubbles in the fluid exiting the slave cylinder.
With a power bleeder, a helper isn’t needed to pump the clutch pedal. The power bleeder is attached to the master cylinder. When the bleeder screw is opened, the fluid flows through the system until it’s clear. Then the bleeder screw is closed and the job is finished.
Some slave cylinders and hydraulic throw-out bearings don’t have a bleeder screw. The unit may have a gold Allen screw, but this is not for bleeding and shouldn’t be loosened or removed. To bleed this type of unit:
1. Push the slave cylinder pushrod inward and slide the plastic retaining strap(s) to one side so the pushrod can fully extend.
2. Tilt the cylinder so the port connection faces up and is at the highest point on the unit. Then fill the unit with brake fluid.
3. Connect the line from the master cylinder to the slave cylinder (lubricate the o-ring with brake fluid and insert the retaining pin).
4. Hold the slave cylinder vertically (pushrod facing down) so the slave cylinder is lower than the master cylinder. Remove the cap on the master cylinder. Then slowly push the slave cylinder pushrod in about one inch. Watch for air bubbles in the fluid in the master cylinder reservoir.
5. Repeat stroking the slave cylinder pushrod 10 to 15 times, or until no more air bubbles are seen.
6. When all the air is out of the system, push the slave cylinder pushrod back in, reattach the plastic straps to hold it in place and install the unit on the bellhousing or transmission input shaft. Do not remove the plastic straps.
When replacing a master clutch cylinder, the unit should be bench bled to remove as much air as possible before it is installed in the vehicle. This can be done by carefully mounting the unit in a bench vice at a 45 degree angle (don’t squeeze the unit too hard or it may crush the cylinder). Fill the fluid reservoir with brake fluid, then use a blunt tool to depress the internal check valve in the outlet port about 1/4 inch. Hold the valve open until fluid flows out. Then release the valve and depress the master cylinder pushrod 10 to 20 times until the pushrod feels firm (no more air in the cylinder). Top off the fluid reservoir and connect the line for the slave cylinder. Put the other end of the line in a clear container that contains brake fluid. Stroke the master cylinder again until there are no air bubbles coming from the line. The line can now be connected to the slave cylinder.