Clutch Master Is In. Why Is The Travel 3/4" Shorter Than The Old One?
05-15-2010, 01:47 PM
#31
Leeroy had the CMC that had the stud in it. I ground off much of the stud in an attempt to make the clutch release. Its better. Most of the time. You need to make sure the clutch pedal doesn't go too far then. The rubber stop in the firewall works fine.
05-15-2010, 01:52 PM
#32
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I have one of PorKen's adjustable clutch pedal stops that I haven't yet installed, if I take more off the stud then I could use the clutch stop to prevent excess pedal travel.
05-15-2010, 05:02 PM
#33
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One thing to remember. Mark Andersons car doesn't have the standard slave. It has a hydraulic throwout or whatever you want to call it. There is a donut shaped hyd. cyl inside the bell housing around the shaft.
01-08-2011, 07:36 PM
#34
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So after a few months of trouble with high rpm shifts, I pulled the CMC piston and had Greg take another 0.080" off of its nose, so it's now .205" or 5mm shorter. Total piston length is now 74.75 mm:
I know that Stan recommends shortening the spring a coil to prevent coil bind- I guess it should be obvious that the compressed spring needs to be shorter than the length of the piston, but the protruding part of the piston is now 17.89 mm. The stock 928 423 173 06 CMC piston spring has a 90 mm free length and is only 14.89 mm long at coil bind (no picture, too buys flinching at the thought of the spring popping out of the vise...)
So- do I really need to mod the piston?
I know that Stan recommends shortening the spring a coil to prevent coil bind- I guess it should be obvious that the compressed spring needs to be shorter than the length of the piston, but the protruding part of the piston is now 17.89 mm. The stock 928 423 173 06 CMC piston spring has a 90 mm free length and is only 14.89 mm long at coil bind (no picture, too buys flinching at the thought of the spring popping out of the vise...)
So- do I really need to mod the piston?
01-08-2011, 08:50 PM
#35
Team Owner
if thats the old spring it should work if its not damaged .
I have seen a few springs that were shattered and the shards were flushed through the system into the slave.
If you use the new spring IIRC it has thicker winds and thus may not compress to bind before shattering.
Put a slot in the tip of the piston so when its fully pushed the end port wont be cut off from the fluid supply
I have seen a few springs that were shattered and the shards were flushed through the system into the slave.
If you use the new spring IIRC it has thicker winds and thus may not compress to bind before shattering.
Put a slot in the tip of the piston so when its fully pushed the end port wont be cut off from the fluid supply
01-09-2011, 02:30 AM
#36
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Thats what I had to do. From what I remeber I need an inch of travel on the clutch rod to get the clutch to fully disengage but only had 3/4 of an inch no mater how many days I spent bleeding the system. So I machined a 1/4 inch off the lenght of the stopper. Which allowed the the clutch master piston an extra 1/4" of travel which translated to an extra 1/4" of stroke on the slave and now the clutch disengages and works perfectly.Don't for get to recut the little groove in the end of the stopper as mentioned earlier.
01-09-2011, 02:54 AM
#37
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Don't for get to recut the little groove in the end of the stopper as mentioned earlier.
02-21-2011, 05:41 PM
#38
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Only took me 6 weeks to finally get the clutch re-bled last night. Went out for a little rear-wheel spin this morning, clutch works perfectly shifting at 6600 rpm, so I guess we're good for now. 
10-30-2011, 12:31 AM
#39
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Excellent thread to answer questions I was having with my clutch problem/s.
My flex hose was being rubbed by the steering flex coupling (about 4 in diam.) and finally leaked.
With the info and pictures from this link was able to get the MC, hose and hard line out today.
http://members.rennlist.com/sharkski...Hydraulics.htm
I was wondering what the difference was between the earlier and later MC.
This thread answered that, no difference except the piston and spring.
Brake fluid was leaking on the carpet and could not find a source for seals but did a few years ago and replaced.
The MC does have corrosion and pits near the top and wore the top seal plus the boot is torn.
I can get a later MC and just use the seals and spring replaced on my piston in that since the casting, bore and depth is the same.
My 1979 928 vin # 9289201425 was the last one made with the 9 inch brake booster.
5 spd and no sun roof. Owned it for 7 years as a daily driver.
My flex hose was being rubbed by the steering flex coupling (about 4 in diam.) and finally leaked.
With the info and pictures from this link was able to get the MC, hose and hard line out today.
http://members.rennlist.com/sharkski...Hydraulics.htm
I was wondering what the difference was between the earlier and later MC.
This thread answered that, no difference except the piston and spring.
Brake fluid was leaking on the carpet and could not find a source for seals but did a few years ago and replaced.
The MC does have corrosion and pits near the top and wore the top seal plus the boot is torn.
I can get a later MC and just use the seals and spring replaced on my piston in that since the casting, bore and depth is the same.
My 1979 928 vin # 9289201425 was the last one made with the 9 inch brake booster.
5 spd and no sun roof. Owned it for 7 years as a daily driver.
11-09-2011, 09:03 PM
#40
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IF there were one available, wouldn't a smaller i.d. SLAVE cylinder give the same effect? Although the clutch pedal effort would increase, the smaller i.d. would cause the piston to move further in the bore, given that it would recieve the same amount of fluid from the master.
07-09-2013, 07:00 PM
#41
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I just recently replaced all clutch hydraulics in my 84.
I must say I am very surprised how well it went. After all the horror stories about how difficult it is to get the master in and out, I must have had some beginners luck.
As the other contributors to this thread, I also made some measurements of the master cylinder for future reference:
Old master Piston stroke made by the clutch pedal before removal: 32mm
Full piston stroke available by new master before modification: 27mm
I grinded off the new piston internal rod 6.5mm
to achieve a total piston stroke of 33,5mm before installing the new master.
I also cut 2 turns off the big end of the spring.
To get the new master in the car, I used a piece of thin rope threaded through a firewall bolt hole and through the corresponding bolt hole of the master.
Since I had no helper, I put some tension on the rope by attaching a rubber band to the end inside the car and tied it to the hand break.
Then beginners luck came in to play!
The master just fit right away. I believe I spent less than 30sec to get it in there, and maybe 3 minutes to get the bolts in.
I then used a pump action oil can to press brake fluid up from the slave all the way up to the reservoir. Then I bleed the system thoroughly the normal way.
All in all, one of the most painless jobs I have done on my 928!
Again,..it must have been beginners luck!
Hvithaien
I must say I am very surprised how well it went. After all the horror stories about how difficult it is to get the master in and out, I must have had some beginners luck.
As the other contributors to this thread, I also made some measurements of the master cylinder for future reference:
Old master Piston stroke made by the clutch pedal before removal: 32mm
Full piston stroke available by new master before modification: 27mm
I grinded off the new piston internal rod 6.5mm
to achieve a total piston stroke of 33,5mm before installing the new master.
I also cut 2 turns off the big end of the spring.
To get the new master in the car, I used a piece of thin rope threaded through a firewall bolt hole and through the corresponding bolt hole of the master.
Since I had no helper, I put some tension on the rope by attaching a rubber band to the end inside the car and tied it to the hand break.
Then beginners luck came in to play!
The master just fit right away. I believe I spent less than 30sec to get it in there, and maybe 3 minutes to get the bolts in.
I then used a pump action oil can to press brake fluid up from the slave all the way up to the reservoir. Then I bleed the system thoroughly the normal way.
All in all, one of the most painless jobs I have done on my 928!
Again,..it must have been beginners luck!
Hvithaien
07-10-2013, 12:27 AM
#42
Team Owner
A caution note,
there is a feed port in the MC bore close to the end of where the snap ring seats,
what you dont want to do is cut off too much of the tip of the piston ,
what will happen is that the rear seal of the piston will travel past the feed port and thus fluid will leak out the rear of the MC when the pedal is fully pressed.'
there is a feed port in the MC bore close to the end of where the snap ring seats,
what you dont want to do is cut off too much of the tip of the piston ,
what will happen is that the rear seal of the piston will travel past the feed port and thus fluid will leak out the rear of the MC when the pedal is fully pressed.'
07-10-2013, 01:07 PM
#43
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I just recently replaced all clutch hydraulics in my 84.
I must say I am very surprised how well it went. After all the horror stories about how difficult it is to get the master in and out, I must have had some beginners luck.
As the other contributors to this thread, I also made some measurements of the master cylinder for future reference:
Old master Piston stroke made by the clutch pedal before removal: 32mm
Full piston stroke available by new master before modification: 27mm
I grinded off the new piston internal rod 6.5mm
to achieve a total piston stroke of 33,5mm before installing the new master.
I also cut 2 turns off the big end of the spring.
To get the new master in the car, I used a piece of thin rope threaded through a firewall bolt hole and through the corresponding bolt hole of the master.
Since I had no helper, I put some tension on the rope by attaching a rubber band to the end inside the car and tied it to the hand break.
Then beginners luck came in to play!
The master just fit right away. I believe I spent less than 30sec to get it in there, and maybe 3 minutes to get the bolts in.
I then used a pump action oil can to press brake fluid up from the slave all the way up to the reservoir. Then I bleed the system thoroughly the normal way.
All in all, one of the most painless jobs I have done on my 928!
Again,..it must have been beginners luck!
Hvithaien
I must say I am very surprised how well it went. After all the horror stories about how difficult it is to get the master in and out, I must have had some beginners luck.
As the other contributors to this thread, I also made some measurements of the master cylinder for future reference:
Old master Piston stroke made by the clutch pedal before removal: 32mm
Full piston stroke available by new master before modification: 27mm
I grinded off the new piston internal rod 6.5mm
to achieve a total piston stroke of 33,5mm before installing the new master.
I also cut 2 turns off the big end of the spring.
To get the new master in the car, I used a piece of thin rope threaded through a firewall bolt hole and through the corresponding bolt hole of the master.
Since I had no helper, I put some tension on the rope by attaching a rubber band to the end inside the car and tied it to the hand break.
Then beginners luck came in to play!
The master just fit right away. I believe I spent less than 30sec to get it in there, and maybe 3 minutes to get the bolts in.
I then used a pump action oil can to press brake fluid up from the slave all the way up to the reservoir. Then I bleed the system thoroughly the normal way.
All in all, one of the most painless jobs I have done on my 928!
Again,..it must have been beginners luck!
Hvithaien
11-07-2013, 01:06 PM
#44
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Since I've never done this and it looks like I need to, I have a quick question for you guys that have BTDT.
Do I need to pull the entire MC out, or do I do this in situ? Also, is there a good way to keep all the fluid from draining out of the reservoir?
Do I need to pull the entire MC out, or do I do this in situ? Also, is there a good way to keep all the fluid from draining out of the reservoir?
11-07-2013, 06:27 PM
#45
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At one point, NAPA offered a rebuild kit for the MC. If you can find one of those, you can rebuild in place. I'm not sure if you could just take the guts out of a new MC and put them in the old body. Maybe someone else can comment. I know there was a thread recently where someone took apart a new MC and it had been changed once again.