| 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.  |