FAQ - Clutch Slave Cylinders
Clutch Slave Cylinder
Q: After bleeding the system, the clutch is still spongy at the lever.
A: Either air is still trapped or air is being drawn in. Or the master cylinder seals are not functioning normally. The master cylinder is the controller of fluid movement, the slave is just a receiver and will only move as much as the levels of fluid received. When air is present, squeezing the lever simply moves that air around, hence no resistance or feel at the lever as the fluid has not compacted to a solid state. This will not only cause gear selection and finding neutral issues, but also increases the chance of other symptoms such as ‘creep’ whilst in gear when at a standstill. And clunky/bad gear changes. The only time a clutch slave can create spongy lever syndrome, is when the seals fail. Resulting in instant fluid loss and master cylinder levels dropping drastically. Which will also make it near impossible to bleed the system properly or if at all.
Q: After bleeding the system, the clutch feels great. But after riding for a while, the pressure begins to fade and the lever becomes spongy. Squeezing the lever repeatedly sometimes or often restores clutch confidence for a short while before becoming spongy again.
A: Either air (or moisture) has been allowed to be drawn in over time or air/moisture was present and is reacting with heat. There is no other cause when heat is a factor. There is nothing in the slave cylinder that will react with heat and change the results in performance. If gear selection with engine running after bleeding is possible, at any point (engine hot or cold), then the slave’s piston U seal is not the cause. Only flushing the system or locating the area (either line in or master cylinder) where the air/moisture is being allowed into the system.
Q: During the installation and bleeding process of the CLU-0116 Ducati clutch slave, the lever is locked solid and the slave’s piston feels jammed?.
A: No, the slave has not jammed, the push rod is either too short or has been made too long during installation. In one case the piston inside the slave is pushing against the silver back plate, hence feeling solid (as the piston has nowhere to go). Or the other case the push rod is now so long, the clutch is constantly disengaged and the clutch basket permanently is being forced apart. It is vital in these scenarios to ensure that the slave is not operated for any reason if not mounted securely to the engine. Otherwise, there are significant dangers the back plate will be forced out of the body and the contents of the slave ejected. This can also lead to extensive damage to all parts within. The extension piece that comes with the kit, is designed to help with versions of Ducati that has a push rod shorter than normal. It is vital that the length of the push rod is established by the use of a Vernier (or a similar precision measuring device) as shown in the installation instructions’ image. Even if it is 0.1mm shorter than 30mm the extension piece must be used, hence accuracy is vital. If the extension piece has been installed incorrectly, then please follow the installation instructions on how to remove it. Or contact a member of Oberon Performance for further guidance. There will be no other reason to why the lever feels ‘locked’. For push rods either shorter than 25mm or longer than 36mm are strongly advised to contact Oberon Performance for guidance.
Q: My CLU-0121 LC8 clutch slave is leaking oil and when I remove the slave from the engine, there is fluid coming out of the breather hole or sides of the silver back plate.
A: If the master cylinder levels are not dropping, then the oil observed is simply not clutch fluid and the fluid coming from elsewhere, more likely from above somewhere. If the master cylinder levels are dropping but gear selection and drive is perfectly fine, then the leak is not from the slave itself. Instead, what is happening is the escaping clutch fluid is reaching the rear of the slave and is being sucked in and blown out. Behind the silver back plate is a cavity and as the piston moves in and out, as it acts like bellows, will fill that cavity over time. And like bellows it will expel small amounts of fluid as the piston moves. If master cylinder levels are dropping over time, and gear selection is difficult or impossible then check the line in, all the way to the master cylinder. Looking for any traces of moisture on the line. Weeps from the master cylinder will travel down the line almost invisible to the human eye in some cases. It will then collect at the end of the line at the back and bottom of the slave cylinder. If no evidence of leaks above the slave cylinder can be found, please contact a member of Oberon Performance’s technical team for further assistance.
Q: My clutch slave is working perfectly but appears to be leaking oil around the bottom or back of slave when I park. But the master cylinder levels are not dropping over time.
A: The leak will not be from the slave cylinder if the master cylinder levels are not dropping. The fluid must be coming from elsewhere as the slave will only leak clutch fluid. There are many places fluid can leak from such as push rod seal and not to mention the action the slave sees from the front sprocket over time (grease/oil). Fluid in small amounts can often look clearer than they normally are. Master cylinders hold such little fluid, that even tiny weeps would empty them in no time. Hence if you’re not topping up often, then whatever it is it isn’t clutch fluid.
Q: My clutch slave is working perfectly but appears to be leaking oil around the bottom or back of slave and the master cylinder levels ‘are’ dropping over time.
A: If you are able to change gear, find neutral and get drive from the rear wheel. Then the leak is not from the internal seal inside the slave itself, it’s practically impossible due to the nature of U seals and how they rely on expansion to work. Nor will they weep over time if the clutch is able to change gears as normal. Fluid is subject to gravity so it will always gather at the end of the line or lowest point, which happens to be the bottom or rear of slave. So, the leak could be anywhere from above all the way up to the master cylinder. The only exceptions to this rule are if the inside bore of the slave has been damaged by an external source such as misalignment or push rod bearing failure. Or there is the presence of debris in the slave that is being allowed to periodically interfere with the U seal on the slave’s piston. In the case of bore damage, the leak goes from light to strong very quickly and likewise for the operation of the clutch goes from ok to useless. This usually ends the lifespan of the slave as damaged bores cannot be recovered and are not covered by warranty as the damage was caused externally. Cases where debris is the problem can only be recognised under the warranty agreement if the unit is returned to Oberon’s factory for inspection and testing. Please contact Oberon Performance with a request for warranty assistance. They will qualify the process to confirm whether the unit has potentially this exceptionally rare issue and to explain how it can be avoided for the future.
Q: Changing gear is no problem, but when accelerating the clutch keeps slipping.
A: The slave will only do what the master tells it to do. If the slave’s piston is not allowed to be pushed back into its home position by the natural force of the push rod. Then something is stopping the fluid being allowed back into the master cylinder. It’s nigh on impossible the slave’s piston has jammed because you wouldn’t have been able to bleed and disengage the clutch plates to change gear. In the master cylinder there is a valve that is designed to push and hold fluid when the lever is pulled in. And when released, the valve is supposed to allow fluid straight back in allowing the push rod and slave’s piston back to their home positions. But if the valve is faulty or contaminated with debris as example, then the amount of fluid is throttled and stopping the piston from retreating. Keeping the plates slightly disengaged and thus causing slippage. Or, the clutch plates are just simply worn and need attention. Only the known history of the bike will help determine the health of the clutch plates and health of the clutch basket and springs as a whole. But overall, the chances clutch slippage is caused by the slave cylinder when gear change is ok is nigh on zero.
Q: After bleeding the system, everything works as it should. But every time after a ride, there is fluid around the base of the bleed nipple. And every time it is wiped and cleaned, it keeps returning.
A: The first thing to watch is the master cylinder levels. If they are not dropping, then there is no leak. One of the main reasons to why fluid keeps coming back is due to the presence of fluid from the bleed residing in the chamber of the bleed screw. When the engine gets hot the fluid that is present will expand and dribble out. After a while, it will cease or you can remove the trapped fluid with an airline. Be sure to take great care and wear eye protection when doing so. If fluid continues to reappear persistently but the master cylinder levels are not dropping. Then the source must be from elsewhere. From where would be impossible to say without inspection so all potential sources should be thoroughly inspected. If the master cylinder levels are dropping, and fluid is seen around the bleed nipple all the time. Then please contact a member of Oberon Performance for guidance and they will ask you to perform a test on the bleed screw. Which is a simple test of removing the screw, and using a marker pen, colouring the end of the screw until it is a solid colour. Then wind the screw back in until finger tight and then ‘pinch’ with a spanner. Undo and remove. Inspecting the angled end of the screw where it was marked with a pen, look for a silver line. A proper seal results in an unbroken line all the way round the base of the screw. This means metal is touching metal and therefore not the source of the leak from the clutch system. If the line is broken then please contact a member of Oberon Performance for further guidance.
Q: The clutch is working perfectly since installation. But now the clutch fluid has darkened with what seems to be engine oil deposits. And in some cases, the master cylinder has been overflowing.
A: For engine oil to push past a hydraulic seal in the slave, up the line and into the master cylinder would require insane levels of back pressure, easily in excess of 120+ psi. This would indicate that something is severely wrong with the engine setup. Oil levels and breather hole malfunctions would be two strong candidates. Oil can be pumped past any seal if enough pressure is exerted, even of gradual amounts over time. In virtually all known cases to Oberon over the last 18 years, has shown that the issue(s) are more than likely to be master cylinder seal reaction. When master cylinder seals are introduced to the incorrect grade of fluid. The seals react in a number of ways and one of them is to secret sludgy oil like deposits that will resemble engine oil. In some severe cases it has been reported that the master cylinder is over flowing with fluid. Master cylinders are supposed to be air tight, therefore to overflow would suggest the unit is simply not sealing correctly. Probably from the seals expanding from their reaction causing fluid displacement which would then cause spillage and overflow. To date and in 18 years of manufacture, Oberon Performance has yet to receive a single returned slave where it has clear evidence engine oil has entered the slave. Each and every one has passed inspection and pressure tests from 0-600 psi and all internal inspections have shown nothing but just clutch fluid. It has been noted though that virtually every reported case has been on the KTM/Husqvarna 690/701 engines. Oberon has tested their KTM 690 in every way and so far, just cannot find a way to force oil up to the master cylinder. So, at present it remains a mystery but suspicions lie firmly on seal reactions from certain grades of oil not compatible with the seals.