C5 C6 Corvette: Why is My Clutch Glazed?

This article applies to the C5 (1997-2004) and C6 Corvette (2005-2013).

Like brake pads, the life of a clutch, by its very nature (friction material), is shortened with use. Friction is created when there is relative motion at contact between the pressure plate and the flywheel surface. Being designed to create friction in order to function, the clutch will naturally be worn down. Yet, some common symptoms of excessive clutch wear include hard/soft pedal, slow or lazy pedal return, pedal hang up mid-travel, pedal sticking to the floor, and no "grab" with high RPM.

Along with inherent wear over time, frequent slipping and clutch dumping can contribute to these issues. Slipping (or feathering) the clutch keeps your clutch partially engaged (as opposed to completely engaged where there is contact, with no relative motion/friction between the surfaces) over an extended, often unnecessary, period of time. Dumping (or popping) the clutch, although usually only occurring when drag racing, refers to high rpm up-shifts or down-shifts, often done with a “slam” down and a “pop” up of the clutch pedal.

At the very least, these driving techniques will introduce excessive dust in the system and eventually damage your seals. At worst, with aggressive driving, you can “glaze” your clutch. Yet, unless you have already damaged your clutch to the point of replacement, all hope is not lost, as there are some things that you can do to extend its life.

The clutch hydraulic system consists of a master cylinder and an actuator. Clutch fluid circulates between these two components. When we speak of “dust” in the fluid, the dust is actually the accumulated friction material that is generated with use. This dust circulates through the system when the clutch is cycled in and out. Of course, it is abrasive and it will eventually damage components such as seals. Also, clutch fluid will degrade. Its water content will boil (emitting steam/gas) at high heat. Degraded and/or “dirty” fluid is easy to identify. In its new state it should be nearly clear. When it is dark and cloudy, it needs to be changed. No amount of clean fluid will heal damaged parts, so preventive maintenance is key.

Changing the Clutch Fluid

Step 1 – Inspect the fluid in the clutch master cylinder

The clutch master cylinder is located near the brake master cylinder. Remove the cap and diaphragm/bladder to inspect.

Step 2 – Draw out fluid from the master cylinder

Using a syringe or turkey baster, draw out the fluid. With a clean, lint-free shop towel, wipe down the reservoir and diaphragm.

Step 3 – Refill the reservoir and replace the diaphragm

Locate the “fill” line and refill the reservoir to just below the line. After you've done this, replace the diaphragm and secure the cap.

Step 4 – Circulate the new fluid

With the engine off, pump the clutch pedal (full top to full bottom) 20 to 30 times. This will circulate the new fluid and blend it with the old fluid. It will also help dislodge any captive residue and displace any trapped air.

Step 5 – Re-inspect the fluid in the reservoir

If the fluid is still cloudy and appears dirty, repeat the process until clean fluid is achieved.

Step 6 – Set up a regularly scheduled clutch fluid change

Even very dirty or degraded clutch fluid can be completely exchanged in about 10 reservoir changes. Once accomplished, depending on your driving habits, you may find that this exchange process should be done every 2 years or 20k-24k miles.

Replacing Clutch Components

Without preventive maintenance or more importantly, with certain driving techniques, you may find that you will have to replace all clutch and hydraulic components. Riding or slipping the clutch to keep up RPM and dumping it to launch may necessitate a complete component replacement.

Unless you have an aftermarket heat tolerant clutch, excessive heating (when you can smell the “clutch burning”) may mean that you have “glazed” your clutch, more specifically, the pressure plate. The abrasive material on this surface heats to such a degree that it solidifies, becoming shiny and glass-like. This reduces the friction coefficient that creates the “bite” and “grab”. In other words, the pedal goes to the floor and rpms rise, but there is no “go” (similar to brake fade).

Sometimes, this overheating does not necessarily mean that you have completely damaged your clutch. A simple “cool down” period (30-60 minutes) may return your clutch to normal operating condition as you have not completely “glazed” the clutch as friction material manufacturers define the term. Yet eventually, cumulative heat will reach a critical point and your clutch will glaze entirely.

Even if you replace all components, if you continue aggressive driving habits and/or do not take preventive maintenance steps, you will find these symptoms and issues appearing again. To avoid this, take these necessary steps:

• Keep your clutch fluid clean with regular changes.
• Keep slipping, feathering, dumping and popping to a minimum.
• Keep launch RPM to no greater than 3600 and use the clutch in one continuous motion.
• Refrain from power shifting.
• If you smell burnt clutch, allow for a cool-down period.