CraigL

Hi everyone,

A few weeks ago I wrestled with a significant clutch drag issue on my 93 GTS. We replaced all of the clutch hydraulics (not fun), bled the h*ll out of the system and the problem persisted. And yes, the proper master and slave cylinders were used.

We pulled the clutch and flywheel for inspection - the previous owner had replaced the clutch, TO bearing, pilot bearing, pressure plate, release arm bushing and had the flywheel resurfaced just 6000 miles earlier. Everything looked new.

I shipped all of it to ClutchMasters (Rialto, CA) for their opinion. They tested the pressure plate and as well as the components and found nothing. They refused my offer to pay them for their time. Wow!

So, we're back to the hydraulics and of course, that pesky clutch master cylinder (CMC) bleeding issue.

Over then next week, the clutch will be reinstalled and we will try several different maneuvers to bleed the CMC. If we are not successful, I am aware that some have found success by shortening the CMC piston stop by 0.200 - 0.250".

If I need to try shortening the piston stop, I am concerned that we might damage the rubber seals on the piston during the machining. I have searched high and low, and have even asked Roger but no one has a rebuild kit for the CMC. So here are a few questions:

1. Any suggestions for locating a CMC rebuild kit?
2. Any creative suggestions for bleeding that pesky CMC? (First remove engine - ha!)
3. The FSM specifies 17.4mm of travel for the clutch slave cylinder piston (pg 30-2). Is this spec valid for both the dual and single clutch packs?

Thanks in advance

SteveG

I've never had one apart, so this is from what I read here. I don't think Clutch Masters can check this with the clutch out. There are 3 T-shaped adjusting pins. IIRC, these set the contact position of the plate and will cause drag. I would get someone to check this before doing anything to the piston.

There is a complete description (several) here, search is your friend.

blown 87

IIRC the only time you shorten the piston length is when using a new CMC on a DD clutch, a 93 will have a single disk unless it has been changed to a DD.

We always bench bleed the CMC before installing it, sure makes bleeding a lot faster.

Rob Edwards

I'm sure that they're all correct, but either you still have a bleeding problem or you've got an incorrect part in there that's affecting release. 928 clutch hydraulic parts are confusing as hell, I think everything below is correct, tho. Anyone sees an error, please point out my mistakes!!

First you need to make sure you have all the right parts for a GTS setup

Single disk clutch 928 116 013 23
PP 928 116 004 16
and release bearing - 928 116 085 08
GTS guide tube- 928 116 087 16
GTS release arm - 928 116 832 09
79-95 clutch master
87-95 clutch slave
87-95 clutch slave rod

(which hopefully you do, but if they were all replaced, just double check...)

There are two clutch master cylinders (CMC), one for 78-79 and the other for 79 (with a 10" brake booster)



The bodies of these CMC's are identical, the pistons and the inlet nipples are the only difference:



Theere are 3 clutch slaves, you want the -22 slave for 87-95:



One other possibility is that the clutch slave pistons are different between early and late (not sure when the change was, I think 78-86 and 87-95:


Late:


Early:



The CMC piston shortening trick is only done to get a 78-86 dual disk clutch setup to release completely when it is installed in an '87+ car that had a single disk clutch originally. The piston is too long and must be shortened to about 74.75 mm in length (at least this works on my DD-equipped GTS) I do not know firsthand whether owners of 79-86 cars have the same problem with incomplete clutch release when one of the above new 79-95 CMC's is installed, and thus whether they need to cut down the piston stop as well. Hopefully someone can chime in on that.


Anyway, for your GTS, if you're running an all-GTS single disk clutch and all the parts are relatively new and check out ok by Clutchmasters, then the problem has to be in the bleeding. If you tried the two-man gravity bleed method and that didn't work, the next step is to try reverse bleeding. Dave Anderson and Stan (MrMerlin) both have excellent writeups on reverse bleeding

http://members.rennlist.com/sharkski...Hydraulics.htm

https://rennlist.com/forums/928-foru...ydraulics.html

I did the reverse bleed on my GT, worked perfectly:




Since your stuff is all installed in the car, you might try to reverse bleed it in situ (connect a motive power bleeder to the slave nipple, pump to 3-5 psi, crack the nipple open, and monitor the reservoir until it's close to full, then pull some brake fluid off and repeat till you've pulled off 10-20 cc) , and then other people have mentioned bleeding the CMC in situ by pulling the piston out just a little bit and 'burping' it. I have not tried this.

Rob Edwards

Since I haven't seen them posted in one place before, on disk 1 of the Morehouse CD's, there's a pdf in the 'transmission' folder that covers the G28 5-speed boxes, published in 1988. There's a page discussing the changes to the S4 single disk clutch, showing a movement of 19.7 mm.



For whatever reason the S4 clutch slave piston is slightly smaller diameter, so if the CMC is the same from 79 to 95, a given pedal stroke should give that incrementally greater slave piston travel.


For comparison, the DD slave piston travel spec is 17.4 mm:

SteveG

Have you bled the piston connected to the pedal at the firewall?

CraigL

Thanks to all who have replied so far.

SteveG: Clutchmasters was able to test the assembly and verified that the pressure plate released properly. We didn't bleed the CMC at the firewall (from inside the car), but that maneuver is on the list of things that will be tried.

Rob Edwards: Thanks for the photodocumentation. And I have ordered the Morehouse CDs. We definitely have the right parts. It looks like [edit: 19.7mm (~ 3/4") of travel is required].

Greg Nettles: Thanks for clearing up the issue of when to consider shortening the piston stop.

Based on all of the above, it looks like we have an issue with bleeding the CMC. The good news is it is a cheap fix. The bad news is obvious: this is a recurrent Rennlist issue for a good reason.

Stay tuned!

Lizard928

I hate the factory CMC!

I put a floor mount wilwood system in my conversion car for this reason. It took 4 pumps of the pedal and the system was bled, no more issues EVER.

However that said, with everything in the car, remove the C clip, and slowly pull out the piston, once the primary seal is nearing the top tip it back and allow the air to escape that way. It is a much better way to do it!

Rob Edwards

Colin-

[dumb question] Which end of the piston is the primary seal? I assume the 'secod' one that is exposed as you pull the piston out of the bore, allowing the CMC to completely fill with brake fluid? [/dumb question]

EDIT: Had a 50/50 shot, pic now corrected.

Mrmerlin

primary seal is the one thats in the furthest,
the secondary seal is the one you will see come out first.
Remove the floormats first then put down rags if you go this route

Lizard928

Stan is correct.

It is the seal on the right. You need to let all the air out of the system. This means you need to break that seal and let the air pass.
This is the only method I use for bleeding the clutch.

ROG100

Buy a real car - an automatic - but that's not a lot of help is it 8>)
Back to my glass of wine.

Lizard928

Hey Rog,

I just dynoed my car after an auto to manual conversion.
Nothing else changed or done, I went from 276RWHP to 303RWHP.
I dont have the charts in front of me, but iirc, I picked up close to 50 RW Ft Lbs too..........

so what were you saying again?

Dave928S

You've probably had too many already ... considering that load of bollocks you just wrote ... ...

CraigL

Wine or WHINE? Speak up. I can't hear you! Ha!!