tw78911sc

I have a 07 Z06, stock clutch, HPDE prepared, at the road course this weekend 2nd or 3rd session, I heel toed at the end of a 160 mph straight into a 90 mph corner, and after the shift the clutch pedal only came back 70%, after a couple of laps I figured out it was only not returning on heel toe down shifts, up shift no issues, down shifts w/o the throttle blip no issue. I dealt with it the rest of the session by using my left foot while accelerating to lift the clutch pedal up with my toes under the pedal, The clutch has fresh motul 600, and completely flushed via a remote bleeder and the fluid is perfectly clear even after the complete weekend. There were some throttle blips that were fine. The clutch never slipped. Ideas? thanks in advance Tom

also I have the one c5 return spring installed. May try removing it all together

NSFW

It's complicated. By which I mean that I've read more about this than I care to admit, and I still don't understand it. For pretty much every theory, there's someone who has disproven that theory. For example, tons of people will tell you it's the master cylinder (and for some is surely was) but in my case I replaced everything but the master cylinder, and I haven't had this problem since.

It got a little obsessed with pedal-return problems after having to pull my C5's pedal up from the floor after doing a heel-toe (at a stoplight, of all places). Here's a summary of what I learned and didn't learn:

https://www.corvetteforum.com/forums...the-floor.html

davidfarmer

remove the helper spring and bleed.

davidfarmer

question, do you hold the clutch down while you are braking, or just hit it quickly after the bulk and of your braking and quickly switch gears? Driving styles vary greatly

Bill Dearborn

In 2015 Owner's of early C6s received a letter from GM extending the warranty on the clutch master cylinder to 10 years. I received the letter for my 08Z and the replacement of the master cylinder resolved my issues with the clutch sticking. I am attaching a picture of the letter I received.

Now that I look at it you probably aren't covered since it is 10 years from the date the car was put into service.

Bill

NSFW

"...if a high pressure pulse occurs during a clutch self-adjustment event."

That got me curious, so I searched, and found this document from the OEM:
https://www.jupojostechnika.eu/pdf/tema%20Sankabos/LUK_susireguliuojancios_sank_EN.pdf

And this:
https://pdfs.semanticscholar.org/71d...766b1e090f.pdf

I wonder if my root cause is somewhere in that mechanism. Like the "adjustment wedge" (page 53 of the second PDF) being pushed outward at high RPM?

tw78911sc

Issue happens when braking with heel/toe. Down shift w/o blip work.

GCZ

Given that you have fresh fluid and no air, I would definitely he looking at an upgraded master cylinder.

Jfryjfry

On page 61 of the second one, it talks about an event that could happen if you are double-clutching while rev-matching:

In some hydraulic release systems, quick pumping of the clutch pedal can actually provoke considerable overstroking. If this cannot be prevented reliably, a mechanical stop (as shown in Figure 17) must be included in the cover during the forming process.

PRE-Z06

What clutch and power level? Also curious what ambient temp was?

NSFW

I installed a pedal travel limiter... still had the same issues. And I pretty much never double-clutch.

Pedal travel limiters are still great though. I'm a little surprised they're any less popular than short-throw shifters.

Z.06

I chased a very similar, if not the same exact problem on my 2008 Z06. The clutch was perfect on the street, but whenever I drove the car on a road course, after about 10 laps, the pedal would go down about halfway to the floor. I would pick it up with the side of my foot and push it fully down and things would be fine again until the next time I got things nice and hot. The problem started when my Z06 had about 25,000 miles on it. While chasing the problem, I tried the following:

* bleeding the clutch using the Ranger method. Many times, to no avail.
* having Chevy replace my clutch master cylinder with the new part, to no avail.
* removing the clutch pedal dual action spring, to no avail.
* bleeding the clutch hydraulics with the snake and syringe method, to no avail.
* paying a Chevy dealer to flush and bleed the slave cylinder, to no avail.

All of this time my friends and acquaintances were telling me to just replace the OEM clutch, pressureplate and slave with an aftermarket unit. Also around this time, the force needed to push the pedal in the first time after lifting the clutch pedal with the side of my foot had increased greatly. So I decided to replace the works with a Monster Clutch LT1S twin disk with new pressure plate, flywheel and slave cylinder when the car had 41,500 miles. The OEM clutch and flywheel that came out of my car looked fantastic. and I could see nothing wrong with the pressure plate appearance either. But I think that the pressure plate must have been binding up somehow when the car got fully hot on track. In any case, the Monster clutch replacement solved by track problem.

BTW, my clutch pedal problem had nothing to do with heel/toe, rev matching, etc. The engine in our cars, the RPM range, and the 3rd gear ratio matches the track so well that I can and often do run entire laps in 3rd gear. So my clutch problem was the same whether or not I was shifting gears or not.

PRE-Z06

The stock C6 (nonZR1) clutch was used behind the LS2 and LS3, the LS7 was pushing its limits. As the disc wears plate load or clamping force from the pressure plate changes, the self adjusters are supposed to compensate for that though can only do so much. Curious if the flywheel and pressure plate were hotspotted showing signs of slippage? As that creates heat and causes the limp pedal from hydraulic fluid in slave boiling or diaphragm springs getting weak, not a good thing either way.

SouthernSon

You mentioned a remote bleeder hose for the slave cylinder. Do you have one of those one way schrader tips on the end? Just a thought, perhaps the threads are allowing air to bleed back in around the tip. Although I have one, I still do the buddy system to close the tip upon releasing. That precludes any possibility of air sneaking back into the hose.

tw78911sc

it is a tikt you close it tight after using. I really doubt clutch is slipping since I was in 4th gear for at least 10 seconds before the down shift. There is a 2000 ft straight that you enter at 120+ and it either forces a quick 5th gear shift or feathering off the last few hundred feet to avoid redline. GPS shows 158 mph before braking and heal toe

PRE-Z06

Stock clutch is having a lot of strain put on it pulling 4th gear to 7k rpm, as the higher the gear and greater the aero drag the worse. I’d recommend upgrading as it’s not worth risking wrecking the car over if not able to engage a lower gear and loose engine braking, I assume you’re not modified?

froggy47

Hi, pardon a question related. I have a 2004 c5z 60k miles. Had the clutch stick down issue, fixed 100% with after market return spring & Ranger flushes. On my 2013 c6z never had issue so far 14k miles.

Question on my 2013 c6z do I already have an updated master cylinder? I also do Ranger method "flush" every event.
If it happens on my 2013, what is my plan? Clutch bites fine, know of at least one issue with the "M" aftermarket clutch on a c6z.

Thanks all, trying to be prepared.

Froggy

PRE-Z06

I don’t know about the updated master cylinder on C6Z, but I know Ranger complained about the clutch pedal sticking at the dragstrip on his C6Z. The heat introduced on a launch has got to be the hardest thing on a clutch, but power shifting or not properly heel and toe shifting also creates heat. Heat is an issue for the fluid, seals and springs as well as friction material. It’s similar to how brakes can fail. I pulled the return spring before adding a remote bleeder on C5, but ultimately if the clutch is not up to handling the heat being created you’re trying to put a band-aid on an issue that needs to be addressed as the “LS7” clutch when coupled behind a heavier Grand Sport with steeper gearing and less torque can handle higher launches yet cut the same 60’ as shown below from my outing Saturday with 5k rpm launch and cut a 1.62 60’

Have cut a best of 1.61 on the 65k mile original clutch, never missed a gear and haven’t removed the spring. Clutch pedal hasn’t stuck in the 3k miles I’ve owned it since I swapped in high temp fluid fwiw. The same clutch is used in the C7 from base to ZR1, it doesn’t have issues at low levels of course as it’s over kill.

NSFW

If you have to do the Ranger thing at every event, I'm not sure that counts as a fix. To me, "fixed" would mean the clutch just works, without new fluid before every track day.

Heat was not a factor in my C5Z pedal-return problems (first time it happened was heel-toe on the way to a stop light), and neither was the master cylinder (still using the same one).

PRE-Z06

The internal hydraulic slave cylinder being exposed to the clutch dust is why you need to bleed the clutch or do the “Ranger” method after tracking as the factory bleeder is a pita to get to. Aftermarket clutches that aren’t organic are going to dust less because they don’t slip as much therefore the fluid will stay cleaner longer. Do you tell people not to bleed their brakes as much if they track the car?