Most/all of the ZF6 clutch slave cylinders produced over the past 5 years or so are defective from the factory. The seals are installed backwards which results in incomplete clutch disengagement, and leaks.

The photos below, starting top-left, show how it's repaired. Bottom-right is the seal installed correctly and waiting for reassembly.

You can potentially damage the seal during this repair. If it makes you uneasy, send it to ZFdoc.com, he'll do it for you for a nominal fee. The snap ring groove may tear it as you're pulling it out. It may be possible to deburr it first, although I didn't try.

The piston is very difficult to remove from the slave. I tapped it against a block of wood to get it most of the way out, then pulled it the rest of the way using some snap ring pliers to grip the cup of the piston.

DO NOT put any tools on the ID surface of the aluminum cylinder. This is a finely machined hydraulic surface and you don't want to scratch it.

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  zf6clutchslave.jpg (209.71 KB)

How do you remove the rubber boot "retainer"? Do you simply pry back the tabs?

Could you apply air pressure to the hydraulic port to force out the piston?

I remember this thread from CF, but I refuse to go back to see how it ended- is there any markings or something to differentiate slaves that have been assembled correctly? Or do they still come off the shelf backwards?

Id be curious to hear about throwing some air to pop it out, also.

Took my chances and didnt reverse the seal on the one I put in last year.

What about the clutch master, any problems there also?

That little star retainer ring popped out easily. It's there to hold the rubber boot in. You can use a screwdriver if you need to, it's a non critical surface. Air pressure should do the trick also. I had to really rock the piston back and forth while tugging on it, which can't be good for the seal.

I don't know how to differentiate defective vs. good, or if there is such a thing as a good part.

The defective seals may have been with just the DOM (drawn over mandrel) steel units. The bulkier cast aluminum(?) ones may not have this problem.

Interesting ...

Scary ...

Nice job, CC!

This is HUGE! THe past 2 slave cylinders, as well as a MASTER cylinder HAD the seal on backwards. I printed the info from the post in the STALIN forums, as well as ZFDoc's site a few years ago and kept it in my helms factory service manual.

To switch the seal is nothing to fear, it's cake, just be careful with the seal.

ALL MASTER and SLAVE cylinders MUST BE CHECKED, do it once, do it right.

Great info, thx!

yes- air will remove the piston easily.
I stuck my air nozzle into the fluid inlet (heh, heh. . .heh, heh) and just barely squeeed the trigger until the piston came out. Just make sure you hold a rag or something over the open end to catch the piston when it pops out.

That probelm is supossed to have been corrected and all defective stock removed, according to GM. Not sure I believe it. I was one of the first to experience slave failures due to the improperly installed seal. I think I went through 5 slaves before this information was found out. The current slave has been on over a year now (knocking on wood)!!

CC, I have to tackle my Clutch MC this weekend. Am I affected by this? Should I be looking into taking apart the MC prior to installation?

No. You have the earlier style hydraulics.

I had the 90 style slave and had it replaced with the 91 style slave with the bleeder fitting on top thinking I would escape the dreaded seal issue. Two years later my slave is leaking again. I fill it once a month. Screw it.

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Get some Trans-X.

I had a '89 T-Bird with hardening valve body seals (according to a Ford shop tech). He said get the stuff.

3 drive cycles later, PERFECT shifting!

Seems the stuff softens and swells rubber.

Ok, but that won't do anything for a hydraulic clutch.

Why would it take two years for the leaking to begin? Wouldn't a reversed seal present itself sooner?

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Hmmm...

???

Are the piston seals made of rubber?

Cheaper durometers (rubber) have properties that are lacking. Buna or natural rubber etc.....it is like anything else, cheaper materials don't last as long as others.

Many are under the impression that Rubber seals are rubber seals........I once had a list of all the different types of Rubber......it listed something like 75 different durometers.

Has anyone had any luck locating a source for a replacement piston seal?
Surely there has to be suitable one out there in the world of fluid power cylinder repair parts.

To the best of my knowledge, there isn't a rebuild kit for these hydraulics.

The fix is to simply reverse the existing seal.

As for "rubber." Most is a silica compound... hell, most car tires are more silica than rubber.

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The brake fluid keeps the rubber plenty soft. The clutch cylinders generally leak due to a scratched or corroded surface.

The ZF6 slaves failed due to the seal being installed backwards, allowing pressure to force fluid past it rather than cause it to expand and seal.