I didn't want to make back to back threads but these issues are so different it made sense.

This spring my clutch has developed odd behavior. My 94 gets driven usually only on weekends. Over a weeks time sitting, the clutch gets mushy and feels like there is air in the system. The car is correspondingly difficult to get into gear and the pedal grabs very low. If I sit there and pump the pedal dozens of times it returns to almost normal. If I drive it, within 10 minutes the pedal is completely normal. The next day it feels normal as well. Sit a week - back to mush.

The master and release cylinders are newer aftermarket units. They are about 6 (release) and 4 (master) years old. The system was vacuum bled when the master was replaced and received all new fluid. Both units are cast pieces as they appeared more robust than the stamped steel ones they replaced. I checked the fluid yesterday and although it is full it does appear dirty in color. I never did replace the braided stainless hose connecting the two.

Any ideas how I am getting what seems to be air in the system? There does not appear to be a leak anywhere. I even pulled the inspection cover and dipped a finger in the bell housing to be sure none was pooling there - also the reservoir remains full. Is there an explanation as to how I can pump the pedal to where it feels normal for a day or two? Where would you start, fresh fluid? Defective master/release cylinders?

 

I would pump it up and hold it a few minutes then slide your foot off the pedal and let it snap back.

 

Can you explain what that would test or prove?

 

When was the last time you replaced the clutch fluid? Could be trace amounts of moisture/and or air. When you pump the pedal after a few weeks you compress the contaminates just enough to give you normal clutch behavior. My guess is the seals in the aftermarket cast units are beginning to deterioate and let air feed into the system when not in use for a prolonged period of time (A week, maybe more). Before any parts replacing, I would get a helper and do the following:

1.Pop the cap on the reservoir and remove bellow
2.Go underneath the car and unbolt the slave, then flip it upside down so the bleed nipple is facing down.
3.Crack bleed screw and as the fluid drops have your helper continue to fill the reservoir with new fluid.
4.Make sure the connection at the master and slave are both "good" and no apparent cross threading or leaking.
5.Re-Assemble.
6..Pump clutch pedal a 100 times with varying speeds.
7.Go have a nice drive
8.Come back in a week or 2 and see what the clutch feels like.

If its still funky, time to replace the master and slave as a pair, either aftermarket again, or give Jim Jandik a call.

 

I was worried that the master/release might be failing. I remember some years back reading about units assembled incorrectly (o-ring backwards) but who knows.

The fluid was last replaced ~4 years ago when the master cylinder was replaced - come to think of it, I believe that was only 3 years ago and probably less than 1500 miles driven since. The slave cylinder was replaced in 2010 I believe.

When the master cylinder was replaced I vacuum bled the system from the bleeder screw on the release cylinder - I presume a vacuum bleed is equivalent if not better than a gravity bleed. I agree, step 1 is new fluid and a bleed. If that doesn't cure it I'll replace master and release cylinders. The units from PTS seem like a sure fit but they certainly do cost a premium - appears to be well over $300 for the pair. Also, I'm not sure that my cast cylinders would even be acceptable as a core to them.

Anyone rebuild master/release cylinders in their home shop? I rebuilt the stock brake master cylinder some years ago with good results. I'm not even sure of the brand of my units now though so sourcing parts would be difficult.

 

It is not a test it is to get the air out. might take a few times.

 

I'd try replacing Fluid and bleeding the system first. If no luck contact Jim at powertorque systems.

 

I think that you're problem is the Master...I think you need to replace/rebuild it. I have the same symptoms on my truck ('96 Silverado/5 speed) and it is the master. I'll replace mine...some day. It's an annoyance, but I deal w/it for now, b/c the truck is an old dog.

 

Same thing a former GM mechanic told me and I've done and it worked. After bleeding the system, still had some air in there somewhere. Pump it several times and hold the pedal for about 5 minutes. The idea is to let any remaining air up to line move up the line to the loop above the MC and collect there. After the 5 minutes, slide your foot off the pedal and the pressure of the returning fluid will push the air out of the line back into the MC, eliminating it.

I did it on mine after having the mc and slave replaced the other year. My mechanic was unable to get all the air out of the system having the same symptoms you described. Did this a couple of times, and it's worked fine ever since.

 

All, thank you for the responses. I plan on replacing the fluid, vacuum bleeding and also trying the hold clutch/slide foot off. I'll report back once it's done - hoping later this week.

 

Last night I put the car up on jack stands, removed the PCM and was about to start the bleed of the clutch system when I realized I didn’t have a bottle of DOT 3 on the shelf. So it didn’t get done, but I did pop the cap off the clutch reservoir again and saw that the fluid is really dark for fluid that is only a few years old and a few thousand miles.

It makes me think that the DOT 3 brake fluid is eating the seals in the aftermarket cylinders and allowing fluid past which may be the cause of my problem. If I replaced the cylinders I may end up with the same problem a few years from now.

It doesn't seem that the clutch fluid needs the extreme boil over protection of brake fluid, its in a toasty area but that doesn't compare to the heat of brakes. A Google of this issue found many people (non corvette) using everything from 10w motor oil to baby oil as the hydraulic fluid for their clutch systems. Anyone try another, more seal friendly, option? I do believe DOT 3 is the correct stuff to use, but I also believe that the aftermarket and likely Chinese made cylinders probably use inferior seals. The OEM rebuilt stuff is just so very expensive.

 

You can't use motor oil as clutch hydraulic fluid. You may be confusing that with trans oil.

The clutch fluid turns dark fairly quickly. A few hundred miles and your new fluid will look murky.

-Moisture
-Clutch Dust
-Heat

^That is what contributes to the dark fluid.

This video is for C5/C6 owners and is called the "Ranger Method".


I do the same thing with my C4 every few months. My car is not an everyday car, so when I drive it, I drive it pretty hard. My fluid will turn murky after a 300 mile road trip.

 

Thank you for the reply and link, which I watched. It perplexes me a bit how dust and water can contaminate a sealed system so quickly. I'm not 100% convinced that the dirty fluid isn't in part seal breakdown.

For a little perspective, my Tacoma brake and clutch cylinders share a common reservoir. At 39k miles the fluid is still a golden color and has never been touched. It's setup is similar to the zf6 in that the slave cylinder is external. Granted it doesn't receive the abuse that the vette does but just trying to make a point that clutch fluid isn't necessarily dirtied immediately by contaminants. And I know - I'm comparing apples to oranges here.

I'll be doing a complete fluid replacement with DOT 3 using a vacuum bleeder. I'll report back with short and long term results.

 

I did the flush/bleed this evening. I used a hand operated vacuum pump at the slave cylinder bleeder while my wife maintained the fluid level in the reservoir. We used Prestone DOT 3 brake fluid and it took about 10 oz before I had perfectly clear fluid out the bleeder end. As a last step I sucked out the fluid from the reservoir and cleaned the cup with q tips. Filled to proper level, checked for pedal and it was very soft but pumped right up. I continued to work the pedal for about 5 minutes for good measure and now it feels perfectly normal. I'll let it sit for a week or so and see how it feels.

No pics of the process but here's the pump I used and the reservoir full of clean fluid. I'll be back in a week or so, with hopefully the same clutch pedal feel. If not, time for new hydraulics I suppose. I think I'll add a clutch fluid flush to my spring shakedown. <br >

<br >

 

Alright guys, I'm back and could use some advice.

1 week ago I replaced 100% of the clutch fluid, bled the system using a vacuum bleeder and pumped the clutch pedal. The pedal effort felt normal.

I let the car sit an entire week on the jack stands and didn't touch it. Today I hopped in and NO clutch whatsoever. I pumped it and the pedal effort came back to normal, took about 30-40 pumps.

There is no fluid on the ground or signs of leaks elsewhere, the reservoir is full and clean. Would you replace the hydraulics at this point? Slave and master or is one more suspect over the other?

If the cylinders are defective, why can I pump them back up to normal? Having not driven or started the car, what transpired to go from normal clutch feel to an effortless pedal that is not actuating the clutch movement?

 

Have had a similar experience, lately, albeit with a 1984 master and RAM hydraulic throwout bearing.

I bought a rebuild kit for the master, hoping that is a fix, as a couple experts recommended, at least as the next step. With no pitting on the cylinder walls, new seals may well be the solution.

Fluid can leak inside the car, under the carpet/pad and not be evident, I'm told.

 

Thanks the reply. Last night I ordered new master and slave cylinders. I'm going to replace both and hope they last longer than the last set.

 

There should still be Clutch Hydraulic Fluid on GM dealers shelves in your area I'd think. GM# 12345347

 

I did debated getting GM fluid. I've always used Prestone brake fluid in the past. I have a new bottle of Toyota DOT 3 on the shelf that I will give a go.

 

After rebuilding my clutch master, I looked for DOT non-synthetic brake fluid. I had Ford Blue and some Valvoline 3/4, labeled synthetic, so I went to my local parts (also hot-rodder for decades) store. (Bob doesn't utter, "what year is it?" much.)

He explained that he cannot source non-synthetic BF from any supplier: one stating that ALL BF has been synthetic for years, tho not labeled such until recently. Further, Bob related that he's run synthetic in his old Chevelle for years, which echoes comments from others that I respect.

I returned home and filled the system with the Valvoline 3/4, then bled the system, but that's another thread.