Corvette Clutch Hydraulics Safety Fix

At some of the longer NCCC autocross events (basically time-trial length) there were several cars (C5 Z06's and C5 LS1's with 6 speeds) that experienced shifting problems from 2nd to 3rd and some, found that their clutch pedal stayed on the floor, and had to be lifted up with the toe of their foot. This is very unsafe when you are trying to reign in 350-405 HP at 80 MPH and possibly find yourself stuck in neutral.

Many of us thought we damaged our clutches or overheated the brake fluid. Evidently this is a common problem Corvettes (C4, C5, C6) caused by the clutch hydraulic fluid not being able to flow as fast as it should because of a restriction put in the tubing by the factory which is supposed to smooth out the clutch operation by causing some slippage, delaying the full engagement of the clutch. When you try and do quick (banging) multiple shifts in succession, the fluid can't return fast enough and the pedal falls to the floor. Normally in a low-speed autocross, we launch in 1st and leave it in 2nd, so we have not seen this before. In drag racing and long autocross (time-trial) courses, like the Colorado State Patrol track, this is when it appears.

I was blaming myself for miss-shifts and Becky was freaking out over the pedal, losing a lot of her concentration. If you slow the shifts down then you will not see the problem, but when you are fighting for 10th's of a second, you will not tend to restrain your shifting.

Some folks on the Corvette and Z06 forums have removed the clutch pedal spring (about a 30 sec job) which normally helps to push the pedal IN, therefore allowing the pedal to return.

Most have success by simply keeping their brake fluid clean by replacing it often. This is easy to do as the clutch hyraulics are self bleeding, and only takes a few minutes. Suck out the reservoir, add new Dot 3 or non-synthetic Dot 4 (like Motul 600) in your C5. The C6's use Dot 4 (GM part# 88958860 (US) 889012440) exclusively. Pump the clutch pedal 20-50 times, suck it out again, add more, pump pedal, and continue to repeat until the fluid is clean and clear. You may need up to 9 passes. Don't get the fluid on any of your painted parts. Here's the kit (Courtesy of Ranger on the Z06 Forum - Keep in a one-gallon Zip-lock bag) you need :
  • Brake fluid for the clutch
  • Sealed can for storing spent fluid
  • Syringe for withdrawing the fluid (MixMizer)
  • Cup filled with paper towels to house the syringe
  • Funnel for use in filling the reservoir

60cc Mix Mizer Oil Syringe
Autobarn $3.99 + $5.99 SH

Evidently the best and permanant fix is to drill out the offending orifice, with an 1/8" drill bit, in the end of the braided tube that connects the clutch master cylinder to the slave unit. The problem and procedure was originally published in Chevy High Performance Magazine and repeated below:

In the Clutch

A Simple, No-Cost Fix for LS1 Camaro and Corvette Clutch Problems
Note: Does not apply to C5's as the line to the slave is bent, not straight.

By Jeff Smith
Photography: Jeff Smith

The clutch comes out, and the Camaro launches hard. The tach hits 6,000 rpm and you stab the clutch pedal, yank on the shifter, and slam the trans into Second gear. Again the motor climbs to 6,000 rpm and you press on the clutch and bang the shifter into what should be Third, but you miss the gear. The motor revs, your foot comes off the gas and you search for Third. On the next pass, the same thing happens. You discover that the only way the trans will shift into Third is if you granny-shift the trans. First to Second is OK, but Second to Third just won’t go.

If this sounds familiar with your fourth-generation Camaro or Corvette, you’re not alone. The LS1 Camaros and Corvettes are plenty quick and have great power. The T56 trans shifts great most of the time, but often it just won’t let you bang Third gear. Several clutch companies claim to offer the solution, but in a recent discussion with Steve Cole of The Turbo Shop, he let us in on a cool little secret that he says will solve these clutch problems and make that T56 shift like a dream.

Ironically, the solution is not with the clutch, but with the hydraulic actuating system. Steve has been building and fine-tuning a high-rpm 302ci LS1 Camaro for Chevrolet that debuted at the SEMA show last year. The Camaro suffered the same fate as the description above and managed to kill a clutch in a matter of minutes. This led Steve to investigate the problem.

The hydraulic clutch actuating system in ’98-and-later Camaros is very simple. It starts with a plastic, injection-molded master cylinder that uses a standard piston and seal assembly to apply pressure when the piston is stroked by the clutch pedal. A –4 steel-braided, Teflon-lined hose connects the master cylinder to a hydraulic slave cylinder that bolts to the front of the transmission and actuates the push-off diaphragm clutch. Steve decided to create a complete bench-mounted system to see if he could locate the problem.

Through experimentation, Steve discovered that it took 5 ½ strokes of the master to move the slave piston through its complete travel. After many hours of experimenting, he discovered a delay in both the actuation of the slave cylinder and the time it took for the fluid to return to the master cylinder. Eventually, this pointed to a restriction in the hydraulic line between the master cylinder and the slave. After a little more detective work, Steve discovered a restrictor in the steel-braided line just downstream of the master cylinder line fitting. By using a pin micrometer, he discovered that the –4 line (which should have an inside diameter of around ¼ inch) included a restrictor that reduced the inside diameter (id) of the line to 0.064-inch or roughly ¼ the id of the hose.

Steve surmised that the restrictor not only delayed release of the clutch, but, more importantly, slowed engagement of the clutch as well. This means that after releasing the clutch pedal when the car is launched, the hydraulic fluid is still trying to return to the master cylinder when the clutch pedal is hit again for the One-Two shift. This again delays the return of fluid to the master. When it comes time for the Third-gear shift, the fluid is still stacked up in the line, preventing the clutch from releasing properly. This is what causes those missed shifts. Slowing the shift down allows the hydraulic system time to recover and perform its normal function.

Once Steve discovered the restrictor in the line, he used a 1/8-inch drill bit and carefully drilled out the restrictor to 1/8-inch, cleaned and flushed the line to remove the chips, reassembled the line to the system, and then bled the system to remove any air. With the system back on the car, a quick test blast down the street instantly eliminated the Third-gear shift problem. Since then, the Camaro has run numerous 12-second quarter-mile runs with no hint of a problem.

So why did Chevy engineers design the hydraulic system like this? Steve thinks that the restrictor has little noticeable effect on clutch disengagement. However, when engaging the clutch, the restrictor softens the “hit,” which reduces the abuse on the drivetrain. On quick, fast shifts, this unfortunately adds slippage and also builds heat very quickly in the clutch and flywheel. For everyday driving, the restrictor may be acceptable, but for performance driving and dragstrip use, this could mean the kiss of death.

This quick fix also applies to older supercars if they employ the stock Chevrolet hydraulic clutch actuating system. Steve’s only experience has been with the ’98-and-newer hydraulic clutch systems, but it’s possible that the older hydraulic systems may also use a restrictor. The only way to determine if the line has a restrictor in it is to slide a 1/8-inch drill bit down the length of the inlet side of the braided steel hose. If the bit hits a restriction, then you know you’ll need to modify the hose. Unfortunately, typical AN –4 lines will not work on this system since the GM line uses special GM hybrid fittings that are not compatible with AN lines.

The best news about this whole deal is that other than investing a couple of hours to remove the line and drill the restrictor, this modification costs nothing. We’re not guaranteeing that this modification will earn you the title of Mr. Six-speed, but it can’t hurt. It certainly doesn’t cost anything to try it.

More Articles about Corvette Clutches

ZR51 performance C4 MN6 Clutch Hydraulic System

Z06 Forum Clutch Pedal Woes


This is the layout of the ’98-and-newer Camaro hydraulic clutch actuating system. The master cylinder (top right) is made of injection-molded plastic and is fitted with a –4 braided-steel line. The line connects the master to the hydraulic throw-out bearing assembly (center) that incorporates a return spring. Pushing on the clutch pedal in the car moves the master-cylinder rod that pushes the piston and creates pressure that moves the throw-out bearing—simple, right?

This is what the hydraulic slave cylinder looks like assembled.

Below the throw-out bearing is a piston that moves inside this cylinder and pushes the throw-out bearing against the fingers of the diaphragm clutch.

Remove the roll pin in the master cylinder by pushing it out using a small punch. The high-pressure line can now be removed.

Note that the roll pin just retains the hose in the master cylinder.

A 0.064-inch pin mic [a #52 drill bit is .0635] is used to determine where the restriction is in the braided-steel line. The end of the pin indicates where the restrictor is in the braided line just beyond the crimped portion of the fitting. Drill this restriction out to 0.125 inch [1/8 inch].

This is the factory hydraulic clutch master-and-slave assembly for a ’93-’97 Camaro. This uses a slave cylinder that actuates a release arm inside the bellhousing that is different from the ’98-and-newer style with the hydraulic throw-out bearing. It’s not clear at this time if these earlier units also have a restrictor in the line.

This is the quick-disconnect fitting located at the hydraulic throw-out bearing that separates much like a quick-disconnect air hose. This fitting can be pulled apart and reconnected without introducing air into the system.

Both Centerforce and McLeod offer high-performance clutch assemblies for the LS1 Camaros and Corvettes but, this would put you out of NCCC stock categories . These offer increased holding capacity and are a worthwhile investment if you’re making more power. But the first step should be to improve the factory hydraulics ...and your car's safety.

text in italics inserted by Mike Konshak.

HTML By Mike Konshak.