996 CLUTCH LINE WITH AIR

Hi all, Got a 996 C2 replaced clutch slave and clutch master cylinder, and bled clutch with Motive power bleeder, worked fine for a few weeks, but now during hard accel on freeway, go through 1-4 fine but 5-6 are hard to shift to. Then after freeway driving at high speed back to stop and go traffic. Clutch has air and difficult to go shift 1-6. As a note I have bled the system 4 times already using the same procedure using Motive power bleeder. No leaks that can be seen on floor and no decrease volume of DOT4. Does the clutch master cylinder reservoir cap have to be completely air tight, could this be the source of air intrusion into the system?

i have seen hydraulic cylinders let in air but would not leak fluid. a scored bore did it......did you replace with new or rebuilt ? brand new good fluid ? a loose cap would not let air in to the system....moisture yes. why did you replace these items ?

I had some experience with difficult bleeding on that system. They are defiently not like the "other" cars. Master reservoir cap does not need be air tight. I'd say, bleed again. Get a whole liter of fluid through system and get it out at the bleeder, you mind find a bubble or two.

Thanks for the input, All the components were brand new out of the box from Pelican parts. I must have bled the system already with 2 liters of DOT4, using the recommended bleed procedure eg... use of power bleeder while clutch pedel is depressed. NOTE 1: Lots of air was bled out during this process, until no bubbles were observed-2 liters of DOT4 used. Any other suggestions to look into would be helpful, in the mean time I'm bleeding the system again.

Did you bleed the brakes as well? I'm not sure if the systems being connected together would have an impact or not...

I believe the two system shares the same reservoir, but each system has it's own master cylinder. Still unsure the cause, its getting annoying.

Hi All again, Here is a update: I just replaced my slaver cylinder again, thanks to Pelican Parts!! Thought that was the problem, but apparently NOT. I have discovered more symptoms to what is going on. First, the hydraulic line fitting to the slave is as tight as i can get it using a flare wrench, but I am concerned that the end fitting of the tube may not seat properly, but its hard to tell. Second and more important, i just came back from a long drive from LA to Palm Springs about 110 miles. The clutch pressure is fine for about the first 30 miles, but I can tell the further and further you go, the worse it got, eventually towards the end of the trip I limped into my hotel in Palm Springs. Mind you I may have only shifted only about 24-30 time since all most all of it was freeway driving. I had to change and bleed the line again at my hotel just to get it working again. And repeat for the return trip same conditions. Could there be vaporization to the DOT 4, causing air in the line.

I wouldn't rule out a defective master cylinder .One test is the check for bubbles in the reservoir when someone pushes the clutch pedal .

OP, when you bled the clutch with the Motive bleeder, did you keep the brake fluid level to the "max" BEFORE you hooked up the bleeder and also kept the level at MAX the whole time during bleeding? The reason I asked is there's an internal wall inside the reservoir that separates the brake and the clutch systems. The wall is not much lower than the MAX mark so if the fluid is below the MAX mark, you can introduce air into the clutch side easily.

The clutch master is new, but I will see if I can observe air bubble when the clutch is depressed. When I attach the motive bleeder, the fill line is almost always WAY over the max fill line. What I don't understand is that it works normal until I drive long drives, and the clutch just becomes soft during these long drives on the freeway. Could it be heat related, because it does not make sense that without even using the clutch it becomes soft. Does not seem to be a mechanical problem..

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I doubt it's temp related as the clutch system is not heated up like the brake hydraulic. Could just be air bubbles take time to migrate. When you bled, what procedures you used? You need to hold the pedal down initially but you also need to hand pump the pedal up and down slowly 10+ times (from memory) or so with the power bleeder connected the whole time.

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+1 That is exactly it. When bleeding the clutch it's better to keep the fluid in the motive bleeder, then using it dry and filling up the reservoir over and over again.

see optional clutch bleed section...

^Yes, that's the factory way and it works very well.

Check the pressure hoses in the clutch line. If the net inside the hose is broken, the rubber might hold the pressure when cold. When hose/rubber heats, it will get soft and expand under pressure. I have seen this kind of damage on brakelines, and there has been no leaks.

Here is my new theory, it has been observed that long drives 100-120 miles WITHOUT to much clutch usage (freeway driving) the clutch goes soft and has the symptoms of air in line, and a clutch bleed is necessary to get it back to normal clutch pressure. This has happened about 5-6 times now, and bleed every time using Motive Bleeder procedures. I pulled the schematic of the clutch hydraulic system, and I see one point of concern that can have the effect that "speedrII" mentioned. It also coincides to my theory, that heat or long drives can induce the air/gas in the clutch line. The component in question is 14, hose with rubber that connects to slave. This hose also is nearly in contact with the coolant return pipe to the radiator. This could be degraded per "speedrII" comment. Question: Is the fluid in the clutch line is primary static and pushes the slave only when clutch is depressed? I don't think the clutch fluid is recirculating, back to the reservoir? Any feedback is appreciated.

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Yes, fluid is stationary when clutch pedal is not moving. Coolant hose will not heat the clutch/brake fluid to boiling point. But it can make the hose soft. If the pressure hose is damaged, you will see it expand when clutch pedal is pressed. If there are no leaks, no air should enter the line. So if there is air, the problem is elswere. Last time when I bled my clutch line, I had helper who pushed the pedal and I opened the bleed screw.

Thanks for the reply, speedrII. I will check the hose as it is depressed and see if there is any damage or bulging when actuated. I have checked up and down the entire clutch hydraulic line system, and I don't see any leaks. I am using a Motive Power Bleeder which was the recommended method over the manual 2 person "push and open bleed screw method".

Hi All, Its been a few months and I have changed the clutch hose by my mechanic. The result is still the same, I am totally frustrated with this situation. The one thing that my mechanic mentioned is that I may need a throw out bearing as well. I do confirm this since, it does have the symptoms of a failing bearing (vibration in the clutch when held down alittle) This has got me thinking about the situation again. Could the throw out bearing be preventing the clutch arm all the way out to push the slave to its idle position, thus over the miles driven (100 miles) giving a sense that the slave only has a little of travel left? The funny thing is, that once bled the clutch feels great. I'm going to bleed it again this weekend and really see if air comes out the line. Thoughts????

This past Sunday I replaced clutch, IMS and RMS, when putting slave back into the trans we cracked the line open to ease it going it. Tried doing pressure bleed but did not get a firm pedal till after doing the manual pump and burp of the line. If you have not done that yet grab a friend and give it a try.

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In my 986 Boxster (very similar systems) I did a brake flush from OEM stock fluid to ATE Blue, but did not flush/bleed the clutch at that time. A few months later after some track weekends I did a complete flush including the clutch, and observed that the first few seconds of fluid that came out of the clutch were blue brake fluid. I didn't expect to see this - I figured I would be seeing the older fluid that predated the blue brake fluid coming from the clutch line. So even though it is not a circular system and the clutch line is a "Dead end/One way", I believe there is some recirculation that goes on. I do not know how rapidly this occurs - in my example several months went between the two events. Steve

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With the car parked, are you able to shift through the gears without any grinding? Did you bleed the entire system including the brakes? If the cylinders weren't pushing enough fluid, you'd have trouble shifting into every gear. Not just 5-6. 2-3 is actually the longest at the shifter. Cylinders letting go doesn't make shifting hard, it makes the pedal action go to the floor and the engagement becomes almost non-existent. There's a few things it could be and air in the system is definitely one. A bad clutch fork is another.