C5 and C6 Clutch Pedal Not Returning - Technically Speaking

James Berry Aug 20, 2014 0 Comment(s)
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Sticky Situation

Q: Hello, my name is Chuck H. and I own a C5 and a C6 Corvette. James, I enjoy your monthly article but I think I have a question for you that may have no answer. My C5 Corvette had developed a problem that I have tried to resolve by performing several repairs and extensive research on the issue.

So here we go; after doing a little spirited driving my clutch pedal will stick to the floor on my C5 Corvette after a few high-rpm shifts. The clutch pedal becomes firm and then sticks to the floor. If I slow down and pump the clutch pedal a few times it returns to normal. The car has actually had this condition for over two years and is absolutely fine on the street under normal driving conditions.

This problem is getting progressively worse. In the past I could always pull the clutch pedal up with my foot and it would work. However; now the pedal is so close to the floor when it sticks I can barely touch the pedal.

Attempts to fix the problem thus far: replaced clutch master cylinder, throwout bearing, and changed the clutch fluid with Castrol Synthetic Brake Fluid (SRF) fluid to avoid a fluid boiling issue since the fluid was a little dark. I have also wrapped the clutch lines in thermal tape.

I was wondering if you had any idea what specifically could be hanging up. Do you have any ideas that may resolve the issue? Have you heard of the drill modification and what is your opinion of this repair? I have read a Technical Service Bulletin that GM has put out on this issue and it seems like I have performed all the repairs that could have caused the fault.

Now for the other hammer to drop; my C6 is in the beginning stages of the same problem. I am in desperate need of advice on this one issue. Thanks for any insight,

Chuck H.

A: Well Chuck, it looks like you have done your homework and this happens to be an extremely common problem with C5 and C6 Corvettes. It’s hard to believe we have not covered this problem before now.

Let’s start with the root problem; I spoke with one of the experts on this subject at Lingenfelter Performance Engineering. They have found that the hydraulic system on the C5 and C6 Corvettes can start to build up heat and the expanding fluid forces the slave cylinder piston to move farther out. Because of the clutch system design, at some point the pressure plate clutch forks don’t have enough spring tension to force the fluid back into the master cylinder. So the clutch pedal sticks to the floor. Remember; the slave cylinder is located inside the bellhousing and this location puts clutch fluid in a very high heat area.

Other symptoms of a clutch hydraulic system problem can include a hard or soft clutch pedal, the clutch pedal to be slow to return to the top of its travel, a clutch pedal that may stick midway in its travel or a clutch pedal that is stuck on the floorboard and will not return.

The most important thing is to keep your clutch fluid clean and fresh so it is important to inspect your clutch fluid. If it is dark you may only need to flush the clutch fluid. It’s recommended to use the Castrol SRF Racing Brake Fluid or an equivalent that is specifically formulated for high heat applications. If you have not had a failure yet it is a good idea to keep your clutch fluid clean and fresh; it is cheap insurance to keeping your pedal working properly.

Installing Clean Fluid and Bleeding the Clutch System

When bleeding the clutch system one of the more difficult things is accessing the bleeder screw, which is difficult to see without using a mirror.

Look where the master cylinder line connects to the slave cylinder and just above the lines at the very top of the transmission you will feel a hex-shaped rod about two inches in length. That is the bleeder screw.

To bleed the system on some models you will need a 7⁄16-inch thin-wall socket. Take care not to drop the socket or it will end up at the bottom of the bellhousing, making it difficult to retrieve. It may be a good idea to tape the components of your tools together. On other models this is much easier to access and you will only need a 9mm wrench.

If you plan to bleed the clutch traditionally, you will need two people. One person under the car operating the bleeder screw, and another person in the car to pump up the pedal and keep the reservoir full of fluid.

Now open the bleeder screw and let the fluid run out until it becomes clear. Be sure to keep the clutch reservoir full so no air can be introduced.

Once the fluid is clear pump the clutch pedal approximately 50 to 100 times with the bleeder closed. Open the bleeder again and look for air or dirt. This may need to be repeated several times until all the air, dirt, and residue has been removed from the system.

After you think all of the air, dirt, and residue has been removed from the system pump the clutch pedal again approximately 50 to 100 times with the bleeder closed, remove the top off the reservoir and let the vehicle sit while you have lunch or a few beers for about 30 minutes.

After the bleeding process is complete fill the clutch reservoir roughly 1⁄16-inch below the full mark so when the clean and dry moisture barrier and lid are installed the fluid will read between the low and high mark.

Drill Modification

Chuck, you asked what the drill modification was. The purpose of this modification is to create a better flow to reduce the shock to the clutch system from rapid movement of the clutch fluid through the lines. GM put a restriction in the line so fluid can’t flow fast enough to abuse the drivetrain. This leads to a sticky pedal or getting locked out of gears in high heat situations.

Procedure

Remove the clutch master and line out of the vehicle. The line is removed from the master by pushing out the roll pin.

Once the line is off, inspect where the line connects to the master cylinder and you’ll notice there is a small restriction in the line.

Use a small drill bit and drill to enlarge the restriction until it is the same size as the line. After drilling into the clutch line remove all the medal shavings by flushing with brake fluid, taking precautions while drilling to minimize the issue of shavings.

Once you’re done make sure no contaminants remain in the brake lines. Use brake fluid and air to flush and blow any chips or shavings out of the line. You don’t want to get metal shavings into the hydraulic system once it’s all back together.

Now for the other problem, the slave cylinder side of the brake line will have a restriction as well and you will need to make the same modification on this end of the brake line using the same drill method. You will find accessing the slave is not an easy procedure. You need to remove the complete drivetrain to access the internal slave in the bellhousing. Not an easy task!

I don’t recommend the Drill Modification procedure because most performance lines, master cylinders and slave cylinders do not have this restriction. If you are going to remove the driveline to access the slave cylinder why not replace it with a performance unit.

Lingenfelter C5/C6 Corvette Clutch Return Spring Kit

Clutch Return Spring Kit 2/9

The engineers at Lingenfelter have come up with a clutch return spring kit that forces the master cylinder orifice to open and allows the fluid to flow back into the master cylinder, which resolves the clutch pedal sticking issue. This kit also speeds up the application of the clutch after the pedal is released. This is cheaper and easier to perform than any other repairs or modifications.

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