Dear Ole Mechanic;Chevrolett Pickup

I was driving my pickup earlier today, and as I pulled up to a stop sign, I stepped on the clutch pedal, and it just went right to the floor.  It didn’t do anything.  Fortunately, there wasn’t anything coming from either direction on the highway.  I slowed down as much as I could with the brakes and hung a fast right turn onto the highway shoulder.  I had to shut the engine off to get it stopped.  The clutch pedal still wouldn’t do a thing.  I checked under the hood, but didn’t see any clutch linkage.  I waited till there wasn’t any traffic and shifted into low gear.  I used the starter to get it rolling and the engine started.  I pulled a "U’"turn and headed back to the house--slowly looking for any parts that might have fallen off.  I got the truck in my driveway and shut it off without hitting anything.

What happened?

Oh, the truck is an 02 Chevy ton with a five-speed manual transmission and a 6.0 V8 engine.  I never had any problems with the clutch before.  Help!

Clutchless


Dear Clutchless;

Have you ever checked the clutch fluid reservoir?  You didn’t say if you had checked it.  Oh well, here goes. 

Your Chevy has a hydraulic clutch system.  The clutch pedal has a push rod that goes through the firewall and pushes the piston in the hydraulic clutch master cylinder.  The clutch master cylinder has a reservoir on top of it and is just to the outside of the brake master cylinder.  The reservoir should be at least-half full of DoT 4 brake fluid.  Only DoT 4 brake fluid should be put in the reservoir.  Do NOT use DoT 3 brake fluid.  DoT 3 fluid is used in the brake reservoir.  Do NOT mix DoT 3 fluid with DoT 4 fluid, as they have different properties and different boiling temperatures.  Using DoT 3 in the clutch can cause a spongy feeling clutch pedal and lead to wear on the synchronizers in the transmission.

With that warning out of the way, I’ll get down to what may have happened to your clutch.  An empty reservoir could have allowed air to get into the system. Since air is compressible, it will give the pedal a soft or spongy feel.  There should be some resistance as the pedal is pushed down; however, if a big slug of air got in, the resistance could be so little that it may feel like there is nothing there.  Check the clutch reservoir.  If it is empty, then fill it with DoT 4 brake fluid and then bleed the system, as you must get ALL of the air out of the system.  That may fix your problem.  The bleeder is down on the side of the transmission bell housing next to where the clutch fluid line goes in.  If you have never bled a hydraulic system, then get someone to help you that has done it before, as it is a two-person job.  It takes one person to open and close the bleeder valve, and another to pump the pedal.  While bleeding the system, keep an eye out for leaking fluid, as it had to go somewhere.  If it is leaking is down the firewall or on the floor inside, then the seal in the clutch master cylinder seal is shot, and the master cylinder needs to be replaced.  No, I don’t think a rebuild kit is available.  If the fluid starts leaking out the drain area at the bottom of the bell housing, then the seal in the slave cylinder is shot, and the slave cylinder will need to be replaced.  To do that, the transmission will need to be pulled out.  The slave cylinder is internal between the transmission and the throw out bearing.  If the transmission needs to be pulled out, then you may as well replace the clutch assembly, as it is probably worn.  You didn’t say how many miles you had on your truck, but a nine year old truck is probably about due for a new clutch.

That leads to another possible problem that does not involve the clutch hydraulic system.  I have see a high-mileage clutch throw the lining off of the clutch plate.  The sudden loss of the lining makes the slave cylinder hit its limit, and it just can’t disengage the clutch.  That would give you a clutch pedal that would have almost no resistance when you push on it.  That is just what you described.  There are a couple of other mechanical clutch problems that could cause the same thing, but there is no need for me to go into them, as they would require the same fix--a total replacement of the clutch assembly (both the clutch disc and the pressure plate).  All of the mechanical problems that I have described require the removal of the transmission, and I would suggest that you get your favorite mechanic involved, as that requires disconnecting the clutch hydraulic system as well as several computer electrical disconnects. 

Yea, even the manual transmission is tied into the computer.

Herr Professor Nuzanbolts