I converted to a ZF 6 speed (from the 4+3) in my '87 vette. The current set up is as follows:

5 7/8" bellhousing (I believe GM also made a 6 1/8")
Fidanza aluminum flywheel
Valeo sprung clutch, pressure plate, t/o bearing
ZF slave and master cylinder (both new)

The problem I am having is that the clutch pedal grabs EXTREMELY low to the floor. I took apart, inspected, and re-assembled both the master and slave cylinders. Both appeared to be in perfect working condition with no leaks and smooth piston movement. I VERY carefully bled the system using the Phoenix Injector tool (several times just to be sure). I am confident that there is no air in the system. The pivot ball stud is in securely.

I took the car to the Vette Doctors and even they couldn't figure out why it was grabbing so low.

I am getting full travel of the master cylinder piston / rod (and slave rod), but it is not enough to completely disengage the clutch at a decent pedal height. It seems like if the master cylinder could push just a little more fluid and get the slave rod (and fork) to travel out another 1/4", it would work.

Any ideas? I ran a bunch of searches and saw a few posts regarding a longer slave rod, but i'm not sure that will solve my particular problem as I don't seem to have any free play in my set up. Won't the longer rod just cause the slave piston to sit farther back in the slave? My system will still only move a certain amount of fluid which translates into only a certain amount of travel, right?

Does anyone sell an adjustable clutch master cylinder for my car (where I could adjust the amount of fluid being moved)?

I am completely stumped! I woke up at 2:30 this morning and started thinking about it. Couldn't get back to sleep for 2 hours!! HELP!!!

 

What is the thickness of the flywheel? What is the F-body/T56 thickness? Did you have the .090" ground off? If the flywheel was meant to be a direct replacement for the ZF6 dual mass, I am at a loss. FWIW, McLeod does sell a master cylinder with a larger piston to move more fluid, but I would think as long as the clutch stack (flywheel/clutch/pressure plate) is the same thickness as the OEM setup, you shouldn't need any more travel.

 

I believe the Fidanza is a direct replacement for the factory dual mass. According to Fidanza, the "step height" is the factory dimension, so I don't think the flywheel thickness is an issue (but you never know).

That's what has me baffled - everything is (should be) a direct factory replacement with factory dimensions. The Vette Doctors had everything apart and confirmed this as well. It figures I wind up with this oddball issue!

If I can't come up with anything else, I think my last hope is a larger and/or adjustable master cylinder.

 

I would check that all your parts add up to the correct thickness--It sounds like something is a little short

 

This is what my mechanic (Carmen at the Vette Doctors - they are very reputable and well known Corvette experts in the northeast) did, since I didn't feel like having to pull the transmission again myself.

He said they measured everything and it all added up to factory specs. He couldn't figure it out. Which makes me not very hopeful that I'll be able to figure it out.

However, since I have this issue, I agree that SOMETHING must not be right. I am just at a loss for what it could be. The only thing I can come up with at this point is that I need to move more fluid to the slave to get the extra travel.

Keep the ideas coming, please!!

 

Wasn’t there a different clutch fork ball that was needed or something. I’ve seen some talk about this in the past, but it’s been awhile.

 

There is a longer (ZR1?) pivot ball stud, but I believe it is used when using the stock single mass steel flywheel from the '85-'88 cars. Since that flywheel is thinner than the dual mass, the ZF6 clutch/PP/TO bearing assembly will sit closer to the engine, hence the need for the longer pivot ball stud in that case.

I've read about issues where guys had problems with the back of the clutch fork actually hitting the pressure plate due to the VERY tight clearances in this set up. If the flywheel and / or clutch is too thick, or the wrong pivot ball stud is used (or if it starts to back out), I can definitely see that happening. But that isn't happening in my case.

I even made a very feeble picture of the geometry of everything (while I'm at work, of course) just to see if something stands out as a possible solution. Needless to say, nothing is coming to my feeble brain!!

 

Is the clutch fork hitting the Pressure Plate, mine did, so I relieved the back of the fork.

Oops, I just saw your last post.

If nothing is hitting then you need to check your slave by having someone push on the clutch pedal while you hold the slave away from the car, try to keep the slave rod from travelling forward. If you can not stop the slave rod, then you system is OK.

If you CAN stop the slave rod, then either your slave or master is faulty or you have air still in the system

 

Did Dennis look at it too?
I've never dealt with Carmen, but found Dennis very very good and easy to deal with.

 

I am going to verify the exact amount of slave piston travel today, and I'll make sure that I am NOT able to hold the piston back while doing so.

Yup - spoke to Dennis as well. He couldn't come up with anything either.

Time to get it up on jack stands yet AGAIN!! AAHHHHH!!!!!

 

When I had this problem, I found out that my Master seals were allowing the fluid to leak by, and in turn not making the slave rod travel far enough.

 

Isnt the Fidanza aluminum flywheel a single mass, or does Fidanza make a single mass that is the same thickness as the dual mass

 

The Fidanza aluminum flywheel is a single mass, but is the same thickness as the oem dual mass. Fidanza confirmed it is a direct replacement.

I'm glad you saw this post, Larry - based on all of my searches, I know you've been through this mess before and probably know the system now as well as anybody!!

When you had this problem, were you using a new master and a new slave cylinder? Both of mine are new, and I THOUGHT the seals, etc. were in pretty good shape when I pulled them apart. But, I will check for leaks on both ONE more time...

How much travel should I expect out of the slave piston / push rod?

Thanks!!
Dan

 

Well - I just checked for leaks (AGAIN) on both the master and slave cylinders. Nothing. I removed the rubber boots and the area underneath the boots (outside of the piston bores) was dry on both. So I don't THINK any fluid is getting by those piston seals. If it is, where else could it go - back into the master cylinder resevoir? I guess that's a possibility...

However, when the clutch is being pressed in and then held at the fully depressed position, I definitely can NOT hold the slave rod back or move the slave piston backwards into the cylinder. It feels pretty freakin' solid. And the slave rod does move as the clutch pedal moves, so I don't think I have any free play in there.

HOWEVER - as for slave piston travel - it only moves about a 1/2"!!!! That seems VERY little. I was expecting 3/4" to 1" of travel. So it seems like the slave piston travel is my problem.

I AM BAFFLED!!!!!!!

 

If it were leaking past the slave seals, you would be able to see the leak in the slave tube behind the boot.

I think it is the Master allowing the fluid to travel past the seal and back into the reservoir,, I could be wrong.

When I tried to hold my slave rod back, I had it on on my knee pushing my palm against it with a rag. I could hold it from travelling forward and you can imagine that the clutch needs much more force than I can hold.

 

I definitely could NOT prevent my slave rod from traveling forward and could NOT push it back in. I had a pretty good angle on it too, so I don't think fluid is getting past the seals.

I just got off the phone with a tech from McLeod. He actually confirmed that 1/2" of travel is all that is required for MOST hydraulic clutch systems, so he felt that my system was functioning the way it should.

After I described the problem I was initially having (clutch pedal grabbing VERY low off the floor), he felt that going with a larger bore master cylinder of 13/16" (stock bore is 3/4") would solve my problem.

I would hate to throw an expensive part like that at it without knowing for sure, but I think I'm out of options.

Any other ideas before I drop another load of cash on this car??

 

Well I did it - I just shelled out $340 for a McLeod (p/n 139022 if anyone is interested) 13/16" bore clutch master cylinder from Summit Racing. It's "temporarily out of stock" and is not expected to ship until 10/2 - 10/4, so I'm out of commission until then.

At least I'll have time to do some other things on the car, and work on my kitchen, until then.

Thanks for all the help troubleshooting!! I'll report back the results once it is installed.

Dan

 

Wow, I used my 1985 Master Slave, maybe I am lucky,,,, NOT!

Good luck!

 

Thanks! Based on all of the testing / troubleshooting I've done, talking to the tech guy at McLeod, and especially all of the online help I've received , I am somewhat confident that a larger bore master cylinder will solve my problem. It's actually a really nice unit that I should be able to keep forever.

Unfortunately, until I have to take the transmission out again (hopefully not for a while!!!) I won't know why my current set up is not functioning properly. Could be just one of those oddball issues that seem to plague us every now and then.

Anyway - thanks again for all of your help!

 

I just remembered one last thing, check the position of the fork on the TOB, if the fork is not shoved all the way toward the center of the car onto the TOB, you will not get enough travel.

I put a screw driver between the Bell housing slave hole and the end of the fork, and shoved my fork over and this gave me more stroke.

Good luck!