NSFW

I have seen a number of threads about this problem, with a few suggestions on how to avoid it, but I haven't seen an explanation of what's going on inside.

Is it the slave cylinder itself that gets stuck, or is there a different component that's preventing it from returning?

Thanks!

------------------------------------------------------------ EDIT ------------------------------------------------------------

Summary: GM says it's caused by an interaction of the master cylinder and the self-adjustment mechanism in the pressure plate.

I solved the problem with aftermarket clutch and I'm still using the same master cylinder.

Some people have good results with the OEM clutch and a Tick (Tilton) master cylinder. Regular bleeding may still be required.

------------------------------------------------------------ EDIT ------------------------------------------------------------

Here we are a couple months later... I decided to replace the slave cylinder, throw-out bearing, and the clutch all at once, and add a remote slave bleeder too. My clutch has been 100% reliable even after track days full of high-RPM shifts.

Because here's what a bunch of reading taught me in the meantime:

Is it caused by the high heat that comes from high power?
This is a popular theory, but I've had this happen when doing heel-toe approaching a stop light. The engine was warm from ordinary driving, but definitely not hot. Coolant was under 200F.

Is clutch slip to blame?
I'm certain that my current OEM clutch is not slipping. I have experienced clutch slip in other cars, but not in my Corvette.

Is it caused by high RPM?
Mine only does it at high RPM, and almost everyone who has the problem says that it only strikes at high RPM. A few people have said that it happens at mid/low RPM sometimes. The weird thing about high RPM is that it creates radial forces, and the slave/throwout move axially. Since these forces are 90 degrees from each other, you have to wonder why high RPM triggers the problem. Something must be happening inside the clutch or throwout bearing to turn those radial forces into an axial problem - but what is it?

Is the pedal spring to blame?
Some people say yes, but removing it made no difference for me.

Is it caused by contaminated clutch fluid?
I rangered the **** out of mine, and got only faint wispy hints of the black goo that other people blame it on. The pedal still stuck the next time I drove. But plenty of people say that bleeding the slave fixes it for them, at least for a while. Eventually the problem returns, perhaps due to the fluid become contaminated again.

Is it caused by clutch fluid leaking past the seal in the slave cylinder?
If that was the case you'd see a lot more reports of hydraulic fluid sprayed around inside the bellhousing. But those are pretty rare.

Is it caused by over-travel of the throwout bearing?
Not in my case - I used a pedal travel limiter to reduce travel until just a hair past the disengagement point, and the problem continued. The pedal travel limiter was a great upgrade anyway though, and I highly recommend it. It feels a lot like a short-throw shift lever - but for your foot.

Is it caused by a restrictor in the clutch hose? (http://www.superchevy.com/how-to/39021/)
Perhaps. I wish I'd seen that theory before I swapped my clutch. I have no idea if the new or old hoses have restrictors, so I don't know if that was a factor or not.

Is the master cylinder to blame?
C6Zs have the same problem, and a bulletin from GM offered new master cylinders up to 10 years after purchase.
Example: https://www.corvetteforum.com/forums...post1600956209

There's also been a case where someone disassembled a C6 master cylinder and found that some internal bits had come apart: https://www.corvetteforum.com/forums...post1589895373

Some C5 owners also report that swapping the master cylinder got their pedal to returns again. Regular bleeding may still be required.

That said... I replaced the clutch and slave, but kept the same master cylinder, and the problem is gone (8 hours of track time so far, no bleeding, no issues). So while a bad master does sometimes cause this problem, other components can also cause it.

Is the clutch's self-adjusting mechanism to blame?
The aforementioned GM service bulletin contains the phrase, "...if a high pressure pulse occurs during a clutch self-adjustment event" which points to the self-adjustment system as a contributing factor. This may explain why Corvettes are plagued with this problem so few others are.

If anybody has a chance to examine the self-adjustment mechanism I'd love to know if you see any features that might contribute to pedal-return issues. LuK (the manufacturer of LS1/6 and LS7 clutches) has published some PDFs about their self-adjustment system, there's nothing in them that screams "root cause" to me. Text and diagrams are no substitute for turning something over in your hands, though.

So what the hell is it?
But I think it all adds up to one thing: there is no one thing. There are at least two problems, and fixing one doesn't solve the problem if other contributing factors are still present. As far as I can tell, the one thing that truly fixes it (without regular bleeding) is replacing the OEM clutch with an aftermarket clutch. Apparently, pretty much any aftermarket clutch will solve the problem. And as long as the drivetrain was coming out for a clutch swap, I figured I might as well replace everything else too, so I'm not sure exactly what fixed mine. But it's definitely fixed. I've logged about 8 hours of track time since then with roughly four high-RPM shifts per minute, and the clutch was the same every single time.

For reference, here are some threads with people discussing theories:

https://www.corvetteforum.com/forums/c5-tech/2041625-clutch-nightmare-to-clutch-heaven.html
https://www.corvetteforum.com/forums...-thoughts.html
https://www.corvetteforum.com/forums...-clutch-3.html
https://www.corvetteforum.com/forums...ky-clutch.html

Venting the bell housing: https://www.corvetteforum.com/forums...ur-clutch.html

Enhanced Ranger method: https://www.corvetteforum.com/forums...er-method.html

More on clutch system longevity: https://www.corvetteforum.com/forums...f-options.html

Installing a TPE remote slave bleeder without dropping the drivetrain: https://www.corvetteforum.com/forums...n-removal.html

More on slave bleeder install: https://www.corvetteforum.com/forums...-question.html

Clutch install guide: https://www.corvetteforum.com/forums...post1592251692

C6 master cylinder failure and repair: https://www.corvetteforum.com/forums...post1589895373

Joshboody

NSFW

It's interesting that it's so repeatable. I've read that it's caused by clutch dust getting into the slave cylinder, but I don't see how that sort of thing would depend on RPM, or get better on its own. I'd expect that to cause more-frequent sticking until it gets bled.

Have you tried staying out of 2nd gear completely, to see if that prevents the problem from happening?

Some friends and I have a Mustang that gets better lap times if we chug along in 3rd for the slowest corners instead of using 2nd... If that avoids the clutch problem, I might do it in my C5 even if it ends up slower. It'll have its first track day at the end of the month, and the clutch thing worries me a bit.

argonaut

I don't know for sure but I suspect it is caused by the dust from clutch plates getting into the slave. I do know how I solved the problem.

When I first started tracking my C5 this would happen. Its been a few years so I don't remember the specifics but as I recalled it was most noticeable at low speeds, even moving around the paddock.

I replaced the clutch (C6 clutch) and while at it installed a remote bleeder on the slave and the Tick master cylinder.

Now my routine maintenance is to bleed the clutch - I do it every few track events and every time the first squirt of fluid thru the bleeder will have black in it from the clutch dust. I have not had a problem since.

So if you don't have a remote bleeder, you better believe the dust build up is bad and I'm pretty certain it contributes to a sticking pedal.

crimlwC6

I'm not sure the exact cause but would also agree that it seems to be related to clutch dust and iirc it is the slave cylinder sticking. It immobilzed my 2008 street C6 after a track day. The 'ranger method' is to use a turkey baster or similar device to suck the clutch fluid out of the reservoir once a track day or so. There are videos where you can see the clutch dust on hard shifts from stock clutch vettes.
As to the stay in 3rd comment, it works! My data shows a shift on a stock clutch costs about a 1/4 second to get back to the speed just prior to the shift. I stay in 3rd as much as possible. My car is under 3k without driver and has ls6 with ASA cam and stock gearing. If the car is over about 38 mph through a corner, I find it better to stay in 3rd. You tend to roll through the corner faster, get on the gas sooner and not muck up a shift down the straight.

vettehardt

I saw it happen to a friend's C5 at an autocross. Pedal went to the floor and stayed there. Got it back to the paddock and proceeded to change the fluid. Used the turkey baster method to remove the fluid in the reservoir then fill what good fluid. Then have him pump the pedal about 50 times. Then remove fluid again and pump agin. Do this about 3-4 times to flush the system as best as you can. He didn't have a problem the rest of the day.

He has been autocrossing the car for about 2 years and had never changed fluid in the clutch.

Adding a remote bleeder to the system would make it easy to fix the issue. Also keeping good fresh fluid in it will do the same.

Joshboody

Sure, staying in 3rd is easier and prob take the corner faster. I will shift to 2nd about <45mph (coupe with longer trans ratios). I like the heel/toe practice... I still fumble a little with an unplanned shift, like inside line going slower than normal. So I like the practice.

I ranger fill all the time. The clutch is a nuisance, but so far I make all my shifts. If this changes, I will swap the clutch and add remote bleeder.

MX621

k24556

Actually, there is an orifice in the master, and it moves slightly in the bore of the master. The purpose,from what I gather is to prevent slamming the pp against the disc/flywheel should your foot slip off the clutch. This came up a bunch of years ago at the Museum, talking to some GM guys about the pedal stuck to the floor issue. So that orifice gets plugged up with a goo mixture of clutch dust and fluid. Then you just bypass fluid and it never pressurizes the slave. I've added remote bleeders to a couple C5's and one C6Z that had the pedal to the floor problem and after flushing all worked. But when you get one that far apart, prudence dictates you swap the slave for new, and a new master as well. The C5 Disconnects are getting old enough that the seals go when you take them apart. Or you can by an AN3 fitting for slave and master and put a flex line in instead of the QD. the fittings are Russell 640281, then a flex line AN3 both ends 12-16 long and no need for the QD. BUT you will have to have a remote bleeder, as the QD's keep fluid all the way to the connection so you can reconnect w/o bleeding (Service Manual, not my opinion)

NSFW

Do Tick master cylinders have that anti-clutch-dump orifice? Or are there others that don't have it?

More importantly, does a master cylinder without that orifice cure the problem?

If so, that would really nail down the root cause.

EDIT: long after writing this, I changed my clutch and slave, but NOT the master cylinder, and the problem went away. So, I'm pretty certain that in my case, the master cylinder had nothing to do with the pedal's failure to return.

k24556

I don't know I just threw them (masters) away and never bothered to take one apart.

I think the Ticks are a larger bore, so the force you have to apply with your foot goes up a bit. Otherwise I think they have a decent track record. don't know if they are constructed the same as an OEM, but I might guess that since they probably use them for different applications besides Corvettes, they are probably a tad different in design/construction. Again if you go that far in, by all means don't forget to put a remote bleeder line in.

One thing I added to my remote line in the engine compartment end instead of a bleed screw, I added a high pressure needle valve. Put a hose barb in the discharge end. Then when it came time to bleed, I hooked up my MITYVAC to the hose barb, pulled a vacuum, then cracked the needle. All I had to do then was keep the reservoir full.

There was a GM bulletin that came out. I attached the one I got for my C6. What they told us was that the "high pressure pulse" was your foot slipping off the pedal, or what us ole geezers call "dumping the clutch"

NSFW

Does anyone know exactly what GM did to the car for that recall? If there is a part number for a new master or slave cylinder, for example....

k24556

it isn't a recall, just a notice, so what happens is they arm-wrestle you to a winner, IF you have the problem. I already had a remote on my C6, so i didn't have a problem. I did have the problem on my C5 (2000 FRC) around 2006, and it was a track car, so I was used to taking it apart, and put a remote in then.

I only know of one person that had the problem described in the notice, his car the same year as mine. he never got the notice, I sent him a copy. At first the dealership would only "bleed" the system (which is a B!#ch, but a tiny bit easier than a C5). That didn't work and they ended up replacing both master and slave. I suggested he ask them to add a remote bleeder line and the dealership refused (which pissed him off but it would have been their liability).

NSFW

Sounds like GM knows that the problem exists but doesn't know how to fix it.

I'm puzzled by the way some people (like JoshBoody, above) report that the problem is RPM dependent, or technique dependent. Makes me wonder if there are two (or more) root causes getting mixed up. Usually when someone has the problem they (totally reasonably) throw the kitchen sink at it with a bleeder, new cylinders, heat shielding, new clutch, etc. Makes me wonder which parts really make the difference. Of course, next fall I plan to do all of the above plus wave a dead chicken over the parts before installation. Maybe sooner if the Ranger trick isn't sufficient.

bellwilliam

another data point: I have an 04 c5 z06. in SOCAL, it only happens at ACS (big ROVAL track) so far for me (haven't driven it at Big Willow yet). It always happens after a long straight.

ranger method do NOT work.

having SRF fluid do NOT work

BrunoTheMellow

Maybe completely different, but my C7 clutch was sticking halfway down because my fluid level was very low.

bigmackloud

This is an extremely common issue. There are almost always 1-3 threads about it on the first page in the C5 tech forum.

Typically the issue pops up after you've increased hp. And the problem happens after several hard, high rpm pulls up through the gears.

I installed the Tick MC. nice unit but not the fix.
I installed the LPE clutch pedal return spring (pulls the pedal back up). Didn't help.
I did the ranger method of changing the clutch fluid before every track event, and like others mentioned, I ran a gear high in the slowest corners.

Even then I would still lose the clutch pedal occasionally. Always seemed to be after several hard 6k rpm pulls up through the gears (like on the back straight at VIR).

The best theory I read once was that the stock clutch is actually slipping just a little with the increased HP and high rpms. That slippage creates a ton of heat inside the bell housing and is actually boiling the clutch fluid momentarily, which is why you lose hydraulic pressure. Give it a few seconds to cool off, and the clutch pedal comes back.

For the vast majority of people, they all say the problem went away when they upgraded the clutch. I don't know of anyone who has replaced just the slave cylinder but stayed with their original stock clutch (not worth it considering the labor), and I doubt that's the answer. I think the additional holding power of the upgraded clutch is the key.

While I've had my engine out for a rebuild, I jumped at the chance to upgrade the clutch. And the Tick remote speed bleeder is worth it's weight in gold. It's a must have item for sure.

Olitho

I don't know exactly what causes the problem from the internal mechanical perspective.

But with over 40,000 miles driving a C5, always with crate LS6 or crate LS3 engines, on race tracks around the country I can share my personal experience of when this happens with the OEM clutch setup as required by some racing bodies.

1. It does not happen very often at low RPM shifts
2. I don't recall it ever happening on upshifts
3. I can almost make it happen on command by shifting immediately as I begin braking for a corner when the engine is still running in the 6000 RPM range. If I am doing 130 MPH like at Auto Club Speedway and get excited and start downshifting immediately then it happens on queue.

Therefore, with the OEM C5 set-up, I do all my braking and then downshift just before turn-in. I almost never have a problem.

And I bleed the clutch regularly through an added bleed line.

I am presuming some sort of aftermarket slave cylinder will probably cure the issue. I have an entire Quartermaster clutch setup from Danny Popp on my GT2 Corvette and I never have a sticking clutch pedal issue.

NSFW

Olitho, can you clarify your #3 point?

It sounds like you're saying you have better results downshifting at high speed / high RPM, but then in the next sentence you're saying that you downshift at low speed / low RPM.

Joshboody

My earlier explanation was NOT so accurate. Let me try again:
- The "light" pedal happens repeatedly on 2 to 3 high rpm shift.
- But actually sticking to the floor only happens on downshift at the track (for me 3 to 2... I don't remember it happening 4 to 3... maybe the higher airflow helps)
- I think the sticking pedal is caused by the high rpm upshift or revving out in 1 gear... but does not happen until the downshift.
- I do NOT remember ever having a stuck pedal on upshifts... but pedal will become "light".

"Light" pedal means... not a consistent pressure like normal. Clutch is more on/off, not a gradual engage/disengagement.

And it does improve by mid-session which seems opposite if heat related.