ROG100

Dennis,
Sorry I meant to say that the size of the booster is a visual indication of the need for an early or late clutch MC.
It would be easier to use the VIN as the split is mid year IIRC.
Roger

Dennis Wilson

Thanks Roger. I haven't had to replace the clutch MC on my early 78 but I'm sure the time will come when it's needed. It sounds like the new or rebuilt units will require changing out to the old spring. That makes it even more tempting to rebuild the old unit from inside the car.

Dennis

Fabio421

Dennis, if you order the clutch master that is specific to the 78 you will not need to open it up. This car was a 79. It should have needed the newer style clutch master but for some reason it didn't. It may have been built in 78 but listed as a 79. We're not sure yet.

Just to reitterate, both styles of clutch master are available.

soontobered84

Fabio,
I feel your pain. I have GOT to be the master of bleeding clutches....When my '84 was being put together for the first time by me, I had to add the missing clutch assembly (master and slave and hoses) I could never get the travel of the slave to be sufficient to allow correct gear shifts or to get it into first gear initally. I think I tried for about 2 months to bleed the dogone thing thinking it must be air in the line. Finally figured out that the travel was not sufficient and the incorrect master was installed. I was seriously bummed finding out that I had to take the damn thing out again.

Fabio421

OK, heres a pic showing both poppets and springs. As you can see. The new style uses the stem on the poppet as a spring stop. The old style allows the compressed spring to be its own stop. Once you remove a portion of the stem you are once again relying upon the compressed spring to be its own stop. The original design worked well for almost 30 years. Thats good enough for me.

Jim bailey - 928 International

What was the part number for the replacement master cylinder ? Perhaps this change in design explains why some people have had such a problem "bleeding out the air" in the two disc clutch setup. With the restricted stroke everything must be PERFECT in order to get the clutch to stop dragging......could it be that simple ?? Ken Osage (Pac NW racer) I believe made the same modification with good results ! Great photos make it very clear !!

OBehave

Exactly Jim!!
I bought a master cyl. from you guys about 2 years ago and didn't install it until 1.5 months ago.I had all sorts of troubles with clutch drag and inconsistant engagement I removed my clutch and checked everything.I even thought maybe, because I waited 2 years to install it that the master was dried up.I bought a new one off you last week along with a pressure bleeder that you so kindly overnighted me so I could make an event this past weekend,and tried that and had the exact same issues.After reading this thread yesterday morning I removed the guts and cut down that stud and re bled it.NO I mean NO drag at all!!! Smooth shifting up and down and lots of pedal travel.
One note though,because the piston travels further be very careful when using the pressure bleeder or you end up pushing fluid into the cockpit.
My car never felt so good!Thanks to Fabio for starting this thread Ed.

Jim bailey - 928 International

That is interesting .."be very careful when using the pressure bleeder or you end up pushing fluid into the cockpit. " Would imply that the brake fluid supply/return port is not in the same location ??

OBehave

Well,I ground off about .687 inches off that spindle and I think that maybe The piston travel is just a tad too long so that you end up stopping at or just passing the inlet port.I was able to bleed it out anyway manually after the dowsing and it works fine.No leaks and no air.
Lets see what happens!

Fabio421

I removed 14mm from the stem. That equates to about .555 " . The old spring measures 12mm when fully compressed and the new spring measures 16mm when fully compressed. I used the old spring to gain additional stroke. I had zero issues with getting fluid in the cockpit. If you put it together under the dash you may have had some resiual oil in the bellows or in front of the outermost seal and that is what you were seeing. I still have the old housing and poppet. I can do some more measuring later in regards to the inlet ports location.

OBehave

[QUOTE=Fabio421;5879252]I removed 14mm from the stem. That equates to about .555 " . The old spring measures 12mm when fully compressed and the new spring measures 16mm when fully compressed. I used the old spring to gain additional stroke. I had zero issues with getting fluid in the cockpit. If you put it together under the dash you may have had some resiual oil in the bellows or in front of the outermost seal and that is what you were seeing. I still have the old housing and poppet. I can do some more measuring later in regards to the inlet ports location.[/QUOT
my car is an '84s and the master cylinder body that is in my car now is the 928 Intl. replacement and I think Jim is right the inlet hose location may be different because of the manufacturing year.The fluid was not residual because you could hear and see it come out under pressure while holding the pedal all the way down.I think I may have taken too much off and plan on fabbing up an adjustable stop on either the floor or the pedal. Ed

Rob Edwards

So Greg and I ran into this problem putting the dual disc setup in the GTS. Of course there's the later style CMC piston in the GTS, and we couldn't get the clutch to completely release. So Greg pulled the CMC piston out and milled off only about .125" off the tip of the stud and put it back together. The clutch works fine now, though strangely the pedal effort is very light- despite having had a beefed up pressure plate modified.

Anyway, I took the car to 928Intl to have Mark thrash on it a bit, and he commented on the extremely light clutch action. When he hammered on it it hooks up fine and there's no slip at all, but he said it's hard to modulate the clutch due to lack of feedback. I guess he's just used to the the elephant Tilton setup in his racecar.

We pulled a pair of CMC's off the shelf- the '78-'79 version (928 423 181 00) and the '79-95 version (928 423 193 00) and took 'em apart. Interestingly, they both use exactly the same housing:



The only difference is the length of the piston and the geometry of the conical portion of the piston between the two seals.



Oh, and the later version comes with a plastic blue hose nipple, while the early one doesn't. I guess it's the plastic elbow on the later unit that allows clearance for the 10" brake booster?

Anyway, we pulled the pistons out and besides the long stud in the later unit, they differ somewhat in the shape of the piston section between the inlet port and the seals. Any idea why the difference?





Mark said that he'd always put the early (78-79) CMC in his S4+ racer with the DD clutch setup and never had a problem. This of course begs two questions- If he's using the early CMC with no stud on the piston and having no problems then we ought to be able to just put the piston assembly from an early CMC into an S4+ car and be ok. (Or grind off the stud as people have done).


The other question: If the 80-86 cars came with the later CMC that has significantly less travel, what else is different about the clutch hydraulics that kept them from having clutch release issues?

I guess I won't muddy the waters with the fact that there are 3 different slaves- 78-82, 83-86, and 87+.....

jcorenman

Interesting thread, and relevant. We recently fitted a DD clutch to our GT, and while it releases fine it takes every bit of travel to do so.

You might want to back off (or remove) the clutch helper-spring under the dash.

And the inlet-ports for the two housings are in the same location? The only issue is uncovering the inlet-port at the bottom of the stroke (as reported by OBehave above, when he took off 0.687" = 17.5 mm).

This isn't helpful, but according to PET there was a CMC part# change in '83:
'78-82 shows 928.423.193.00 (late '78, w/10" booster)
'83-95 shows 928.423.173.06

I think the only difference is rod length, to accommodate the different release arms and PP's. You need the slave for a 83-86 for your clutch, or at least it's rod. (The DD clutch rod is 138.7mm, single-disc clutch is 154mm-- see this thread).

Iwanna928

Glad you got it sorted out! Happy shifting!

Rob Edwards

Jim-

Good question- I can't say yes with 100% certainty 'cause I didn't measure that, but the casting # on the CMC body is the same on both CMCs, and the overall depth of the machined bore is identical.

I didn't think to measure more dimensions on the pistons themselves. I have a late CMC here at the house I can dissect and will post the overall length, stud length, and span between the two seals.