Hydraulic clutch problems

[Roy748Bip]

I don’t have a problem with the weight of the clutch lever but I hate having to bleed the clutch every couple of hundred miles. The last ride i went out the lever came back to the bar but pumped up enough to disengage the drive after a frantic moment:o

As the slave cylinder looks original (1995 bike!) and all I’ve done to it is change the piston and seal (ongoing problem), will a new one fix the problem? Did they ever change the design, or should I buy one of those uprated units? Any suggestions would be welcome (the cheaper the better!)

[Mr C]

I have a simular problem and it turned out to be the bearing that is in the presure plate it had seazed up, This caused the push rod to spin and of course spun the piston which in turn boiled the Clutch fluid and badda bing no clutch. Cost about £7 to fix

[crm250]

As Mr C said, had the same here and ruined a new lightweight unit only 1 week after replacing.

[sparkin]

I got an uprated STM unit off Nelly at the BMF,absolutley fantastic,next to a slipper clutch it`s one of the best mods you can do,highly recommended:D:D

[Old Yella]

Agree with everyone so far.

I had the same on the 996 with the bearing seizing.
Also used the uprated slave cylinder to improve the clutch pull.

If it is a seized bearing it could also have worh the inside on the slave cylinder where the seal seats in so might not be/wont be a good seal for the piston seal against the cylinder?

Worth replacing if you can get one of the uprated slaves but the bearing would be my first check.

[TopiToo]

Hello

Roy748Bip I have/ had the same problem with the master cylinder,
and I made the mistake of replacing the master cylinder without checking the bearing on the pressure plate, as a result the pushrod was spinning and scored the cylinder resulting in a useless master cylinder, so if you do go for an oversized master cylinder make sure the pressure plate bearing is running freely, or for about £6 replace it. Maybe also check the pushrod as it maybe rubbing/wearing down. Take it to a dealers and ask them to compare.

good luck mate and keep us posted.

regards

TopiToo

[Edited on 15-07-1968 by TopiToo]

[Roy748Bip]

Thanks for the info, I’ve had this problem since I got the bike last year. I changed the pressure plate bearing (which was seized!) at the same time as putting the new seal and piston in the slave cylinder.

This cured the problem for about 5-600 miles but it came back with a vengeance. I’m thinking I may change the fluid to DOT 5.1 to increase the boiling point, I’m assuming the seals will be OK with this, has anyone else used this?

[rockhopper]

Sounds like the cylinder itself is knackered. Get a whole new one, either standard ducati or a lightweight one from Cornerspeed.
In my opinion trying to rebuild the old one is a false economy as the bore gets corroded and soon chews up your nice new seals etc. The type of fluid shouldnt matter.

[RickyX]

I had this on my 748 - bearing replaced, seals replaced, cylinder replaced....it still didn't cure it!
Then..fitted a bleed banjo bolt at handlebar end and that cured it! Basically it's a pain getting air out of the line at the handlebar end if you haven't got a nipple there like on the later (brake) cylinders. Installing a Goodridge bleed bolt allows the air to be bled once and for all....IMHO.
R X

[sparkin]

Originally posted by RickyX
I had this on my 748 - bearing replaced, seals replaced, cylinder replaced....it still didn't cure it!
Then..fitted a bleed banjo bolt at handlebar end and that cured it! Basically it's a pain getting air out of the line at the handlebar end if you haven't got a nipple there like on the later (brake) cylinders. Installing a Goodridge bleed bolt allows the air to be bled once and for all....IMHO.
R X

That`s true,the only way to bleed it properly without a banjo bleed bolt is to tip the bike on it`s side or remove the lever entirely,thou some people loosen the banjo fitting,never done it that way myself.

[rockhopper]

Must be something odd about the 748 then! I got a new standard slave cylinder and it bled up easily with only the standard bleed nipple on the slave its self.

[Shazaam!]

There's a common problem that mimics having air in the clutch hydraulic system. The clutch pushrod is spinning because the bearing in the pressure plate is not moving freely. This causes the pushrod to move the clutch slave piston back into its bore slightly.

So, when you ride for awhile without using the clutch, when you try to use it, you have to pump the lever a little to first move the cylinder back into position, after which it works normally. Low-pull aftermarket slave units accentuate this problem because a full lever stroke moves the slave cylinder piston a shorter distance than it does the stock unit.

You usually just need to lubricate or replace the pushrod bearing on pressure plate to cure the problem.

If you are experiencing problems with clutch actuation only during periods of prolonged high temperature operation, such as during a track day, or on a hot day where your temperature gauge is reading 220+ºF, you’ve likely got water in your slave cylinder. The water will boil when it reaches 212ºF and since the water vapor (steam) is compressible, the clutch behaves as though there’s air in the system.

Water/moisture can be found in nearly all hydraulic systems. It can enter a system in several ways. One of the more common, is by using old or pre-opened fluid. Remember that brake fluid absorbs moisture from the surrounding air. Tightly-sealing hydraulic fluid bottles, and not storing them for long periods of time, will help keep moisture out. Also, when changing or bleeding brake fluid you should always replace the master cylinder reservoir cover as soon as possible to prevent moisture from entering.

So, condensation (small moisture droplets) can easily form in the lines and slave cylinders, just like in brake systems.

In the brake system, caliper and line temperatures heat up and then cool repeatedly, condensation occurs, leaving behind an increase in moisture/water. Eventually, the moisture becomes trapped in the internal sections of calipers, lines, master cylinders, etc. and when this water is heated by friction in the brakes to 212ºF, it turns to steam. This steam will create pressure in the system, sometimes to the point that enough pressure is created to push caliper pistons into the brake pad. This will create brake drag as the rotor and pads make contact and can also create more heat in the system.

In a Ducati hydraulic clutch system the engine temperature at the slave cylinder location can easily reach 212ºF (the boiling point of water), and nearby small pockets of water, as they boil, introduce water vapor in the line that causes behavior just like air in the line does.

You’d probably assume that only enough moisture from the air can be absorbed to saturate the hydraulic fluid to it’s wet boiling point, (around 300ºF for a DOT4 spec fluid) and conclude that the fluid (saturated with water) wouldn’t boil because it couldn’t get hot enough.

Well, that’s not the case.

The amount of water that hydraulic fluid can hold in solution is dependent on the temperature of the fluid. So, the Ducati clutch hydraulic circuit will behave like a little water generator. When the fluid temperature rises at the slave cylinder from engine heat, the fluid can hold more than five times more water than it could hold at room temperature. If the fluid is saturated with water at room temperature, it will absorb still more when heated.

Further, the fluid cools down when you shutdown so the additional moisture that can be held at the higher temperature, condenses out into water droplets that sink (water is heavier than hydraulic fluid) to the low-point of the system - to the slave cylinder. This happens over-and-over until enough water has accumulated (at the hottest point in the system.) Somewhere above 212ºF it boils, and your clutch actuation goes south.

The only solution to this is to flush your system to get rid of the water-saturated fluid and the water droplets completely. That means getting rid of all the fluid in the reservoir and in the lines, and particularly make sure that any water droplets that have puddled in the slave cylinder are flushed out. Use DOT 4 fluid from a sealed container and make doubly sure that the reservoir cover is sealing properly.

A badly-sealed reservoir may be the root cause of your problem.

Then bleed air from the system, preferably from it’s high-point. 749/999 have a factory bleed nipple located inside the fluid reservoir to make it easy. Older bikes are notoriously difficult to bleed because the factory bleed nipple is located above the system low-point on the top of the slave cylinder. If you replace the banjo bolt at the master cylinder on earlier models with one with a bleed nipple it’s much easier to bleed air from these systems.

[sparkin]

Originally posted by rockhopper
Must be something odd about the 748 then! I got a new standard slave cylinder and it bled up easily with only the standard bleed nipple on the slave its self.

Probably would Rock,if the reservoir didn`t draw any air in,but if you change hoses it's a bit of a bugger.

[Roy748Bip]

Jeez Shazaam!, a couple of lines would have done! But seriously, thanks.

I think you may have hit it on the head there, it was a very hot day for Scotland (28 °C) and I was tanking on a bit. I think there must have been water in the system that boiled because there was no warning when the lever went.

I think I’ll invest in a new standard slave cylinder, new Goodridge line and a bleed nipple at the master cylinder at some point. I really appreciate the help you guy’s give out on this board because I’m running the 748 on a shoestring and do all my own maintenance.