C6 Clutch and Brake Fluid

**Larry M.** · May 18, 2015, 07:41 PM

Re: C6 Clutch and Brake Fluid

Originally posted by Jimmy Gregg (2756)

I had always heard that C-6 Corvettes were prone to clutch failure if you did not change the fluid. I thought it was BS until now. I just changed the fluid in the MC and Clutch. MC was simple and basically the old fluid was still fairly clear. The clutch on the other hand was an entirely different story. I used a turkey baster to empty the reservoir 5 times, pumping the clutch 50 times in each rotation. Much better and cleaner now and after taking a test run, darn if it does not shift better. So for $8.00 and 20 minutes of time I may have prevented a nightmare. So be warned C-6 owners This is a simple job and needs to be done, and frequently. Hopefully, I have attached a photo of the old fluid and remember it has been diluted by 500% with new fluid.

I agree. For my 2011 GS car, I simply replace the fluid in the reservoir about every 3-4 months and then simply drive the car. After doing this a few times the reservoir remained clear and now stays that way.

Larry

**Patrick H.** · May 18, 2015, 07:55 PM

Re: C6 Clutch and Brake Fluid

I did mine in the 2008 late last summer, then again this past Wednesday when getting it ready for the Joplin trip, then tonight when I went to pick it up. Each time it takes less swapping to get it to come clear.

**Duke W.** · May 19, 2015, 09:15 AM

Re: C6 Clutch and Brake Fluid

I think the fluid and elastomers in the clutch slave cylinder take a beating from heat. I learned decades ago that changing fluid every couple of years will vastly extend the life of hydraulic components by preventing corrosion. My '88 MBZ 190E 2.6 five-speed brake system has only required two new pad sets and a parking brake adjustment since new, and I attribute that to bienniel brake/clutch fluid changes.

What you guys are doing certainly helps, but the best way is to bleed through fresh fluid every year or two on a daily driver.

It's my understanding that this is a time consuming job on a C6 because it requires removing the bottom panel of the center tunnel to access the slave cylinder bleed valve, but I recall reading that some outfit offers an extension pipe with a bleed valve that you attach to the bleed valve thread on the slave, which places the new bleed valve in an accessible location in the engine compartment. You have to remove the tunnel bottom panel to install the extension pipe, but you only have to do it once.

The Corvette engineers deserve a trip to the woodshed for not making clutch slave cylinder bleeding an easy job.

Duke

**John D.** · May 19, 2015, 05:22 PM

Re: C6 Clutch and Brake Fluid

There was a bulletin on this issue...

#PIP4938: Discolored Dirty Dark Sludge And/Or Grease In The Clutch Fluid Reservoir - (Aug 4, 2011)

Subject:	Discolored Dirty Dark Sludge and/or Grease in the Clutch Fluid Reservoir
Models:	2004-2012 Cadillac CTS CTS-V
2000-2012 Chevrolet Corvette,
2004-2012 Chevrolet Corvette Z06
2009-2012 Chevrolet Corvette ZR1
2004-2006 Pontiac GTO
2007-2009 Pontiac G8
Equipped with manual transmissions

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern:It may be observed on some vehicles that the clutch fluid in the reservoir appears to be discolored, dirty, have sludge or grease contaminates in it after being in service for a period of time.
The level of discoloration will vary with factors such as driving conditions, under hood temperatures etc.
This is a phenomenon that may occur with time due to the interaction between the clutch system rubber parts and hydraulic clutch fluid. Comments have been received about the fluid in the hydraulic clutch system being discolored. This discoloration is the result of carbon black used in the seal manufacturing process leaching into the hydraulic fluid used in the clutch system. The discoloration may also collect on the inside of the clutch reservoir at the top of the fluid. This discoloration does not affect the operation of the clutch system and should not be considered a reason to flush the clutch hydraulic system
The small amount of sludge or grease that may be noticed is, in fact, grease that has migrated from the master cylinder assembly. The grease is used during assembly of the master cylinder.

Recommendation/Instructions:

**Edward J.** · May 19, 2015, 05:54 PM

Re: C6 Clutch and Brake Fluid

I think the engineering dept maybe correct in a sense, but over the coarse of years, this black sediment will certainly cause problems down the road. these carbon fragments cannot be good for the bores along with moisture internally of the slave or master cylinders, I myself have seen a few failures of masters and slaves, no picnic for C5/6/7 owners when they fail.$$$$

**Jimmy G.** · May 19, 2015, 08:15 PM

Re: C6 Clutch and Brake Fluid

I think that Service Bulletin may be a CYA as my clutch works better with new fluid

**Paul H.** · May 20, 2015, 06:25 AM

Re: C6 Clutch and Brake Fluid

I routinely do the fluid evacuations of the clutch fluid on my 06 z06. You can feel the difference in the clutch performance when you do it. I agree with Ed that all that crap in the fluid is not good for the system.

**Patrick H.** · May 20, 2015, 10:19 AM

Re: C6 Clutch and Brake Fluid

It makes you wonder why some sort of filter wasn't added to the system between the clutch MC and the actual clutch so that all the dust and garbage could be kept out of the MC. One would think that it could have been possible if GM had wanted to do this.

Anyone know if the C7 system has improved at all?

**Edward M.** · May 20, 2015, 10:42 AM

Re: C6 Clutch and Brake Fluid

A friend had the master cylinder fail on an 09 that had less than 10K miles on it. Car is not driven regularly and the clutch pedal just went to the floor one day, and stayed there. Changed the fluid with no result. Car went to the dealer, where they diagnosed a bad clutch master cylinder.

Service manager explained that the dust from the clutch was getting past the slave cylinder seal and contaminating the entire system. He said that the dust had no way to escape, so it gathered on the slave cylinder rod, then got pulled into the slave cylinder, and from there to the master cylinder.

He has done a lot of these repairs. Don't know if his explanation holds up, but .....

**Duke W.** · May 20, 2015, 10:59 AM

Re: C6 Clutch and Brake Fluid

GM continues to deny, as it has for decades, that periodic brake/clutch fluid changes are a good and necessary maintenance practice while European manufacturers have recommended it for decades. Historically, their primary concern was the lowered boiling point from moisture contamination, which could lead to loss of braking ability when slowing from unlimited autobaun speeds was required.

Elastomer wear particle contamination and solids from the glycol-based assembly grease are issues, however, the bulletin does not address potential corrosion damage from moisture contamination, which is a primary reason periodic fluid changes are a good idea, especially if you plan on keeping the car for a long time.

I recall after a few years my SWC developed a brake problem, which proved to be a plugged compensating port from the sludge at the bottom of the master cylinder. I cleaned out the sludge, but did not completely bleed the system and in 1968 when it was only five years old the master cylinder failed.

It wasn't until 1981 when I upgraded the brakes on my Cosworth Vega to ventilated Monza rotors and calipers, inspected the rear wheel cylinders and found signficant corrosion that I got religious about brake fluid changes.

I changed the fluid every two years on my '88 MBZ since new, and for the first couple of changes the fluid darkened rather quickly, but then remained relatively clear. I expect the initial wear particles and assembly grease solids, if any, took a couple of flushes to be completely expelled, and after time elastomer wear probably decreased as the components bed in their mating surfaces.

I have no doubt that bienniel brake fluid changes are the reason it has only required a couple of pad sets and a parking brake adjustment since new... same with the Cosworth Vega since '81 - nothing but fluid and front pad changes - six to be exact because it acculumulated about 5000 miles of race track hot lapping from the early eighties to the late nineties, but the rear drum shoes are original.

If you plan on keeping a car a long time, change glycol fluid periodically or rebuild the system using silicone fluid, but silicone fluid is not recommended for modern ABS systems.

Duke

**Duke W.** · May 20, 2015, 12:57 PM

Re: C6 Clutch and Brake Fluid

Originally posted by Edward McComas (9316)

Service manager explained that the dust from the clutch was getting past the slave cylinder seal and contaminating the entire system. He said that the dust had no way to escape, so it gathered on the slave cylinder rod, then got pulled into the slave cylinder, and from there to the master cylinder.

That's a possiblity, but what evidence led to that conclusion? Also, if true, periodic flushing would likely have helped, and if true the design is deficient. And what about the possiblity of corrosion?

Any C5/6 owners who have to replace clutch master or slave cylinders should get the parts back, disassemble them and inspect for corrosion and debris.

In the meantime consider flushing the system.

Here's an interesting story. About 10 years ago the clutch began to drag on my '88 MBZ, and when I removed the slave cylinder the attached pushrod that bears on the clutch fork was worn and mushroomed. Disassembly of the slave showed the walls to be essentially brand new - no corrosion and no debris.

Bleeding an empty clutch slave can be difficult. The sure fire way to fill it up and purge air is to loosen the slave bleed valve about half a turn, then attach a bleed hose to a front caliper bleed screw. Then bleed the front brake calliper in the conventional manner, which will force fluid into the slave and up through the clutch master to the reservoir, which will quickly fill the system and purge air. The brake reservoir on the Merc also serves the clutch, but the inlet is well above the bottom, so if fluid is being lost, the clutch will quit working before the brakes fail.

Before I replaced the slave I flushed the brake system (It was about due.), so the clutch fluid was also fresh.

Duke

**Don L.** · May 25, 2015, 08:16 AM

Re: C6 Clutch and Brake Fluid

Interesting thread. Good information, guys. Thanks.

I had heard a bit about this issue in the past, so when I had my 2010 Grand Sport coupe to the dealer for regular service (while still under warranty), I reported the discolored clutch fluid and got the "ah, it's fine" reply. I let it go. Now I'm reading this and it causes me to re-think. See the photo I took this morning of what my brake fluid and my clutch fluid look like after 4.5 years and 5,600 "street only" miles. Notice the difference in color.

Clutch and Master Cylinder Fluids May 25, 2015.jpg

I see the dismissing GM Tech Bulletin and read all that's posted here so far and figure a simple change in fluid can't go wrong, therefore, I'm going to change it out.

Question #1: any recommendations on the best fluid to use for this job?
Question #2: will simply drawing out the old fluid and replacing a couple/few times, until clear, be as good as a full drain and replace?

**Duke W.** · May 25, 2015, 08:59 AM

Re: C6 Clutch and Brake Fluid

Use a DOT 4 fluid. Siphon all the old fluid out of the reservoir and use Q-tips to clean any debris from the bottom. Do it again when the fluid darkens. Flushing the system by bleeding the slave until the fluid is clear is best, but it's a time consuming job as I understand the center tunnel bottom panel must be removed to access the slave cylinder. See post #4.

Duke

**Terry M.** · May 25, 2015, 11:29 AM

Re: C6 Clutch and Brake Fluid

I use GM Clutch & Brake Fluid (I can get the part number from the garage if you want). I had to buy a case of it when I first got the C6. It came in a metal can back then, so I don't worry about deterioration. I am told it now comes in a plastic container. If that is true you are better off with any DOT 4 fluid from your FLAPS. A popular FLAPS with a high brake fluid turn-over is best for the freshest fluid.