guys! i was reallly drunk or somethink last night! i was thinking the reservior had two chambers for the brakes and the clutch. i totally forgot that the clutch master was that thing buried in the fender . so, that makes even less sense how under WOT, that the clutch master and slave. (separate system cept for the blue hose that joins the fluid to the clutch master, right?) SO under WOT, it would have to be heat?? or vibration?? but it only does it after a hard acceleration, not redline if gingerly approached.(tried this). so, its got to be heat or maybe the speed at which i put the clutch pedal down. too faster and maybe it sticks . Ill have to try that tomororw. then, its off to the parts bin for a slave change, and maybe ill figure out a way to replace the master. yuk! i remember now, we did scots by cutting the hole in the fender well. I think i have a master that i bought many years ago. time to put it in I guess. (as long as the S4 is the same as the 84 i bought it for)
thanks guys. sure, i know that replacing everything will fix the issue, im just puzzled on why its doing what its doing . Its a "me" thing. im just like that.

 

you havent seen a 928 before have you. thanks for the info but 928s are TANKS. there is no flexing of anything on a 928 chassis its so friggen strong in so may places its crazy!! however, all the connective stuff could be suspect. ive blown out clutch pedals, even broken one in half. blown out the bearings and the connective shaft for the clutch and brake. Its all new on the holbert car, but the chassis is a little tweeked.

I think i have a bad master, so ill try and tackle it this winter!

 

I have seen this happen before when there was excessive vibration in the clutch assembly / TT shaft. At lower RPM, there is no issue, but RPM over ~5k and the clutch does not come up as strong or all the way until it is pumped 2 - 3 times. Try revving the car to 6k RPM in neutral and slowly depress the clutch 2 - 3 inches and see if it you feel any excessive vibration.

Dan
'91 928GT S/C 475hp/460lb.ft

 

The reservoir does have two chambers with a dam in between them.

Heat and vibration when you are in one gear for a longer time - like a higher gear at the end of the straight.

You could also have a pinhole leak in the flexi or hard lines between the m/c and slave. Having experienced both this would give the symptoms you describe.

 

..

 

I have to take issue with this statement just slightly, Mark. (Here we go) And point out something any newby will note the first time they put a jack to one side. And to potentially save someone's windshield. If you raise one side you will probably not be able to open or close the door on that side, so it does flex. And it is good procedure to NOT raise the car w/o the cross member in place or you may crack the windscreen. If it is on ramps or both sides are level, you can remove and replace. Probably obvious to everyone anyway, just thought I'd mention it.

The car is a tank compared to a 944 or most other vehicles so I don't really know what the 944 observation has to do with it. Mysteries abound and certainly this clutch problem is one of them. Curious to see what the answer is.

 

that makes sense, because that clutch lever at the belhousing, could vibrate, push fluid back up to the master and then be flat . hmmm, ill have to check that out, but i did the 6000rpm shift with no WOT, and it shifted fine. only under rocket acceleration does the problem occure.
thanks.

Yes, the two chambers are for front and rear brakes or is for brakes and clutch? i think brakes and clutch, but i dont think that matters much as the point about the pin hole leak is what is concerning, but that feeds the MC for the clutch., but as long as the clutch MC has fluid in it, why would a rapid push at high rpm make any difference whether i was at 6000rpm WOT or 6000rpm without being floored. puzzling.

yes, its still a chassis ,and it has soft points. yes the cross bar under the hood should be in place, but the chassis is unbelievealbly strong. you wont have that sheet metal flex and issues with mounting points of clutch and brake in the firewall that was talked about with regards to the 944. yes, when the cage is installed, the overall stiffness of the chassis is improved 10x, but all the component subframes and firewall, etc are built like a tank. this is why the 944 can have this kind of issue. like the M3 BMW back in the late 90s, they had to reinforce so many points so that things didnt rip out of the chassis.

 

Ok, I did some tests, and as you all know, sometimes recollecting things can be difficult, if you dont really narrow the scope of the test down and pay attention. I started to get out and run the tests at part throttle, shifting after redline and on a decell first, and then found that just the RPM in neutral and a depression of the clutch pedal , caused it to not come back fully. whats happening, is that the vibration of the clutch pack, at that high rpm, generallly past 5000rpm, puts a pressure load on the hydralics that it cant support and the fluid is backed through slave or master and the pedal is not pumped up to the normal height. However, what if there is air in the line, that never got removed because as a side note, when you guys started talking about the fluid change, heat from the headers ,etc, i went to investigate and found the clutch hard line almost leaning on the lower driverside header!!!! i mean right on it., so , i pulled it back and strapped it back 3 " to the chassis. problem solved right????? bled the system, by pushing the piston back in the slave and put it back on. (easy cheating way to bleed) . thinking that this would now solve the issue, i went to test. NO DIFFERENCE. lines are totally away from headers now. its something else.

also as a side note, for some reason, when i pull the slave off the lower clutch bell housing, and the piston shaft, doesnt extent much. in fact, I can push it in for fluid movement by only an inch or so. normally, when you remove the slave, the piston shaft moves slowly forward and you need to reallly push the slave to the belll housing with the shaft in the clutch arm holder, and push the flluid back up to the master and reservior, and then quickly start the mounting bolts for the slave. mine is not like that. i can just attach it,, no pushing needed. (maybe 5mm) and then assembly very leasurly. this seems very very wrong. the slave is the same as the holbert, the lower bell housing is the same (replaced wiith mark A. stock of the identical version) the only wild card is the master cylinder that came on the chassis.

still, whatever the problem, it hasnt been one in 3 years of racing. only just after the last race. so, im thiiking the best first step might be to put the pressure bleeder on the reservior and then open the bleeder on the slave and see if this is just a big air bubble that is causing the problem.

there was a long period of time, that after a race, the pedal was very soft. still was at the top of travel, but the clutch wouldnt be totally engaged until a bunch of depressions. Like, heat caused air bubbles after shutdown and stayed there until the next day or week startup. but, with some pumping, it would come back and full engagement of the clutch would occur.

Ive always had bad luck in the bleeding technique of opening the bleeder on the clutch and then pushing the pedal. it just falls to the flooor and then by luck, eventually it comes back. (gravity, luck, whatever).

 

From your description, I'd have to agree that there is something "uneven" that is "pounding" the T/O bearing, fork, and slave at higher rpms.

When the "blue hose" gets changed, there is a very common mistake that people make, time after time. The hose must run downhill all the way from the reservour to the master, which means it must be routed around the "outer" side of the brake booster...at about 3:00, if you were able to see it from the front.

Most everyone leaves the hose too long and routes it "under" the booster....at the 5:00-6:00 o'clock position.

If this occurs, there is always air trapped in the section of hose that drops below the level of the master cylinder, which is very difficult to ever get rid of completely. This will result in the system "sucking up air" even when one thinks it is perfectly bled.

This is not your problem, but I thought I might shed some "light" for others....and for you, if you decide to replace that hose.

 

So, what do you think the problem is? could a little pocket of air be doing this? why would the shaft be so shallow and so easy to feed into the bell housing and in the "arm" hole and mount the slave. normally, its a little bit of a challenge to hold it against the spring and fluid pressure. hmmmmm what causes the fluid pressure. the master? the slave? I mean, the slave bolts right up. that doesnt seem right. maybe something bent in the crash, as the bell housing was pretty tweeked. maybe it brent the throwout fork slightly.

ahhh, heck, its been working for years now, and why all of the sudden. Ill try a a bleed. is the best way to just put a pressure bleeder on it and force out the fluid, or do you need to press the pedal down too, (press and release while opening and closing the slave nipple ilke doing brakes??)

 

Your symptoms all seem to point to air in the system, other than a few other remote possibilities. The consensus on here seems to be that bench bleeding is the only way to go. I've never done it that way, but I've been convinced by reading threads here that it probably is the most effective method.

 

Bleed the clutch comprehensively to make sure there is no air in the hydraulic system. If the problem still occurs only when the engine is revved past 5k RPM, it's probably a problem with the slave cylinder allowing air bubbles into the line due to driveline vibration. A new slave cylinder will solve the issue for some time, how long depends on how bad the vibrations are.

Dan
'91 928GT S/C 475hp/460lb.ft

 

+928

 

Any thoughts about a "pinned" intermediate plate possibly contributing to unusual vibrations ??? It is a deviation from stock and how it was designed to work. Possibly wear on the disc at the flywheel now has the springs somewhat overextended ? No idea what that could do.

 

the pinning should be totally independant of balance, or vibration. the 3 little pins are very light in weight and are in the 3 positions around the interm. plate. where on the discs wont extend the springs. the springs just press the plate reaer ward when the clutch is depressed. the H's are the stops, so the only thing the movement of the H's change, is that they can move forward based on pressure plate pressure to allow the discs to stack up and press on the flywheell. as i mentioned a long time agoi, when i changed the discs after 7 years of racing, there was no wear on the discs. you could see the part number ink on the discs themselves. you would have to have a lot of wear before the H's needed to be moved. thats why i pinned them. figure other things will wear out first before the discs wear enough to cause the Hs to need to be moved. and if they did, the only effect that would cause would be clutch slip. nothing in the assembly is effected by the pinning of the H adjusters. only that they cant be moved now. all other things are working just as if it was stock. (i.e. springs, int plate, the rear stopping effect of the H's, etc)

I dont notice any unusual vibration, but there is a little more than i remember on the Holbert car. im thinking a small amount of runnout is now present on the end of the crank from the accident. all that weight 27lbs, plus the driveline attached, all just bollted with those little screws at the very end of the crank. something might be off a few thou now. but again, nothing was an issue for racing this thing at redline for the past 3 years, so i think its really got to be the air in the line or a slave/ master issue. again, i have never really had very good luck bleeding the clutch. it doesnt make sense that you cant just do it like brakes. anyway, so i always avoid it and leave sleeping dogs ........ (well, they woke up, so i got to fix it.)