Clutch disengagement problem

[syrarch04]

After spending the winter bringing my new Z10C back to life I finally got to drive it Saturday. It ran fantastic, with one exception; the clutch was having problems disengaging once it was in gear. I'd come back from a run and even with the pedal pushed in it wasn't releasing, so if I stopped, it stalled. It didn't happen every time, but probably 80% of the time. I had rebuilt the master cylinder (Girling), but not the slave cylinder, so maybe that's sticking. There also appears to be some sort of adjustment possible on the actuator lever with the spring from under slave cylinder. I haven't touched that yet, cause I'm not really sure what it does.

I asked the previous owner about it, and they never had any clutch related problems.

Any ideas/pointers?

[Brian]

Two initial thoughts. There should be a return spring from the clutch arm up to the slave cylinder, to pull the arm up all the way. You may not have bled the slave cylinder enough, sometimes you need to undo the bolts and tilt the slave cylinder so that the bleed screw is pointed vertically to get the last of the air out.

There is always the possibility that the pushrod needs to be adjusted, of course. You need to adjust so that you get full disengagement, and then adjust the stop nut so you can't over-throw the clutch.

Brian

[kea]

First thing you need to check is the push-rod adjustment at the clutch arm. There should be a very small amount of free-play. If it gets much over "a rattle", you will start giving up travel. Also, on the Mk series, the clutch arm at the gearbox, has been know to bend at the end. This needs to be solved (modified), or once again, you will lose the ability to dis-engage the clutch. A normal problem is with the slave cylinder itself. Because it spends most of the time, with their bore exposed (piston all the way back at rest), pitting can occur and with it some clutch dis-engagement issues.
I'm not a fan of the spring to the arm and lack of one does not cause the problem you have. The (spring) fingers of the clutch cover, have more than enough force to return the throw-out bearing after you release the clutch pedal. And what little load remains on the bearing has little effect on the bearing life as we use them. Dirt and oil leaking on and around the bearing, has the greatest effect on shortening it's life.

[syrarch04]

Ok, so the latest: I disassembled the old slave and there was a minor amount of pitting on the bore, so I replaced it with a new one. It may have been rebuildable, but at least now I can rule that out if it reoccurred. I bled the lines again, and took it out to an auto-x the next morning. On a quick test drive it worked great, so I lined up at grid, clutch still disengaging fine. I did my run and when I swung back around to grid the problem came back! I tried pumping the pedal in case it needed some pressure built up, but it didn't make a difference.

I did notice that some fluid had overflowed through the top of the master cylinder cap (Girling .87) for what that's worth. Maybe that's because of the push start (popped clutch in second to start) I needed for my second run? Pedal travel feels a bit "two stage", for lack of a better term. First half of the travel takes less pressure than the second. I had bled the lines until no bubbles were seen so I thought I got all the air out, but I didn't try tilting the slave like Brian suggested above. Still some air maybe? I have not adjusted the position the end of the pushrod end, or the lever with the screw, since it was working fine for the previous owner. Picture of that set up below for reference. The lever does not seem to be bent. Is it possible air gets into one of the fittings without any fluid leaking out? Everything's dry except that overflow at the master.

I will try bleeding again with the slave tilted, but if I don't see any air, what should I try next?

[bob darcey]

The "two-stage" feel is because the "first stage" is merely taking up the free play between the throwout bearing and the pressure plate. I think you need to adjust the rod between the slave cylinder and the lever arm to be longer. If you grab the rod and push it vertically upward, it will move the slave piston to the top of its cylinder--this is where you want the piston to return to when you let off the pedal. Then push the lever downward to where you can feel the throwout bearing touch the diaphragm in the pressure plate. Adjust the "bullet" on the rod to where there is just a bit of free play left before the throwout bearing is solidly against the diphragm, and lock the bullet in place with the jam nut. That should give you full motion on the pressure plate and get the clutch to release. Then, you will need to adjust the "stop" in the system to prevent overcentering of the diaphragm. The stop may be on the pedal, or it may be on the rod, but you will need to limit the amount of travel to just what is needed to disengage the clutch. When you are finished, double check that there is free play. The worst-case scenario is that there is no free play and the throwout bearing is actually pushing the crankshaft forward against the thrust washer--it will destroy the thrust washer and mess up the crank pretty quickly if there is no free play.

[Albatross]

Inside the case: sometimes one or both the arms that cradle the throw-out bearing begin to break away from the shaft; as stated previously: the spring attached to the lever outside the case (as you show in the picture) is totally unnecessary! The whole original system was not designed to take the load of a "racing" type clutch cover. That is why the annular is the way to go.

[kea]

Slave bore pitted.
That being the case, you would be wasting time and money trying to rebuild it. I would hope you would not put new seals in a pitted bore and they do not make over-sized seals for a bore that has been honed clean (larger). Replacing it is the smart move.
Sure is a large size master (.87") for a clutch.

[Dave Welsh]

I had the same issues issues with my T342 when I first started autocrossing it in 1986.

It felt to me that the fluid was aerating in the slave cylinder, especially at low speed events.

I replaced the slave cylinder with a new one, and still had the same issues, no pedal upon exiting the course after a run.

After taking the slave cylinder apart, and looking inside it, I discovered that the bleeder is not at the very top of the cylinder when it is orientated with the push rod pushing down. The slave cylinder was designed to push in a horizontal plane, not a vertical plane. As such, there is a large air pocket at the top of the slave cylinder while the push rod is in a vertical plane.

My cure was to drill and tap a 1/8 pipe thread on the end of the slave cylinder, and install a pipe plug into that hole. The final step of bleeding the clutch is to crack the pipe plug while pressing the clutch pedal to evacuate the trapped air. Problem solved. The clutch has worked perfectly since.

[Don Becker]

Replace the master cylinder. If the slave was pitted then the master has had the same fluid and is probably bad too. They are cheap and easy to find so its not worth spending a lot of time rebuilding one that likely has a pitted bore. The fact that the clutch works initially seems to rule out broken forks and other mechanical components. You mention that the master is spitting fluid out the top so that also tells me something is wrong there.

[syrarch04]

Keith, sorry I misspoke, the slave is .87 (new), the master is .75 (rebuilt).

Dave, your problem sure sounds exactly like mine. I'm thinking there is air trapped at the top of the slave. Is it possible that if there is in fact a pocket of air there that the pressure on it could force fluid back through the lines when the pedal is released, resulting in the overflow at the master?

When I took the master apart to rebuild it, the cylinder bore looked great, with zero signs of pitting. If it didn't look as nice as it did I would have replaced it for sure.

I will attempt to evacuate the possible air at the slave, and then move on to things like replacing the master, etc. The problem is I need to test drive to see if it worked and I'm having trouble finding a suitable location. Time to start begging for permission from parking lot owners!

[Dick R.]

The problem is I need to test drive to see if it worked and I'm having trouble finding a suitable location. Time to start begging for permission from parking lot owners!

I use 6" by 6" timbers as "jack stands". These are secure enough and low enough that I can "test drive" the clutch and shifting with the car running in the garage. Use the brakes lightly to "load" the wheels to verify. May not work for your problem if it is time/heat sensitive but with a bunch of clutch cycles on blocks it might be OK.

FYI with my anular slave cylinder one part of my clutch bleed process is to hold the clutch pedal depressed (working alone) with a "quick clamp" and then loosen the bleed screw at the slave. That might help you. Plus, can you unbolt the slave and orient it as needed to allow any trapped air to be bled? I also use a speed bleeder to allow one person bleeding.

Dick
85 VD
CM 85

[syrarch04]

I can indeed unbolt the slave, it mounts with two screws, so that's easy. I have speed bleeders on the front brakes, but don't have one at this location yet. I'll have to measure the screw when I get home to see if speed bleeder makes a screw the proper size.

[kea]

I'm guessing the bleeder size is 3/8"X24, also common on many calipers.

[syrarch04]

Update time. I unbolted and tilted the slave so the screw was pointed upward and rebled. I went for a test drive yesterday, and it's working perfectly! Such a relief. Thanks for all the help guys!

[Brian]

" You may not have bled the slave cylinder enough, sometimes you need to undo the bolts and tilt the slave cylinder so that the bleed screw is pointed vertically to get the last of the air out."

Hmmm = my suggestion from the very first reply to this thread...

Glad it worked out for you.

Brian

[syrarch04]

Yes, it was your advise that worked (and Dave Welsh), you are a super genius. Thanks Brian and Dave!