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AO, I just replaced the clutch MC and blue hose on my GT a couple weekends ago. After reading through as much as I could, and staring at the area to work in, I finally just bit the bullet and pulled the brake master and the booster to get to the clutch MC. I think it may be a pain either way, but at least I could see what I was doing. Had a good friend help me as well, makes a big difference having an extra set of hands and eyes. Just take your time and it's really just nuts and bolts... and a bit of limbo/yoga under the dash... not as easy as it used to be for an older fat boy like me.
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?
The brake reservoir has a divider in it that keeps all the fluid from draining out. If you remove the blue hose it will only partially drain down, probably designed to keep you from losing brakes if you have a clutch hydraulic failure. Just let it drain until it stops, then top it off when installing the new hose and bleeding.
Guys thanks again for all the write ups. Sure enought this was the issue.
Luckily, a PO (not naming names) cut an access panel for the master cylinder. You would never know its there until you remove the fender liner. Here's what it looks like. I'm not advocating you do this, but if its already been done, why not take advantage of it!
I pulled the master and measures the piston. 80mm
I took a grinder and removed about 5mm. I chamfered the edge and put a small slot on the end like it was. The I lopped a couple coils off the spring.
I then bled the clutch. I couldn't quite get all the air out. When you see the angle of the master and how the blue hose guests routed, it's no wonder. I figured if I could raise the front of the car high enough, it would burb itself... It worked. Two pumps of the pedal and I had a nice firm cutch.
Best of all, I can now put the car into first or reverse when it's cold. Thanks again boys!
from your access picture it appears the blue hose isnt up enough thus its possible that it can trap air as an another idea run the blue hose to the engine side of the booster this may change the angle enough to provide a rise to the feed port
This is a great thread and very informative. I find myself needing to make the very same modification in my application which is a dual disc clutch with a new clutch master cylinder (CMC). In reading the thread above, it appears we can safely remove 5mm off the piston without concerns about the piston traveling past the fill port and leaking into the cabin. Additionally, it's recommended that we cut a couple coils off the return spring to ensure the travel does not over-compress the spring. All the threads on this topic have some good insights on what people have done but it was hard for me to visualize what what going on and what were the ranges/limits to amount that should be removed from both the spring and piston.
Fortunately I had a spare CMC assembly and I thought I would take some measurements for form my own clarity. Below is a cross section of the CMC with measurements. You will note the critical distance in this diagram is from the front of the fill port to the bottom of the bore. 60.5mm. The forward seal on the piston must not pass the front edge of the fill port to prevent hydraulic fluid from leaking past the seal and into the driver footwell.
Here is the same diagram with the unmodified piston installed, also note the measurement of the fully compressed spring
Some observations:
The fully compressed spring is 17.1mm, but the piston extension (spring guide) 23.8mm, meaning the spring is never fully compressed it has about 6.7mm of expansion. Here is what the spring looks like fully compressed to 17.1mm versus partially compressed to 23.8. So when shortening the spring and piston, it would seem you would want to preserve this partial compression at full stroke without sacrificing enough spring force to allow full return.
Piston length relative to fill port at full compression.
The measurement of the unmodified piston from the based on the piston extension (spring guide) to the front seal is 70.3mm and the measurement from the leading edge of the fill port to the bottom of the bore is 60.5mm. That leaves a 9.8mm distance between the fill port and the seal. This measurement proves that the recommended 5mm modification is will within the safety margin. My question here is can we go further to ensure even more clutch diss-engagement? It would seem removing 8mm would still be safe.
CMC travel impacts on clutch movement
Under the car, in reviewing the clutch fork travel the unmodified travel is 17.4mm. (measured via the inspection port). This is not enough to release the front disc, even with all the fiddling in the world with the H-adjusters. An additional 5mm of travel is much better and I can turn the disks by hand but there still is some light scraping. Adding a few more MM of travel absolutely remove all friction. There seems to be plenty of available travel in the fork/guid tube, release bearing to allow for an addition 8mm of travel. I wanted to get some thoughts before I go head and make the mods.
Here are some additional photos of measurements I made. The red pant on the piston and spring represent the 5mm/ 2coil mods
red paint represents cut marks for modification tape approximates bottom of bore, note the front fill hole (right) Closeup of fill port
FYI, I. am able to precisely move the clutch fork and take measurements with the lower bell-housing removed by activating the clutch fork with a tie-down strap attached between the clutch fork and the rear cross member. Measurements are taken from the starer ring reference
FWIW the reason 5 MM was used is because thats what the size of the original piston was and 5mm more is the additional amount that new piston stop has.
Shortening the piston to 75mm MM from 80 mm should give plenty of release distance to the clutch if everything else is properly matched and bled.
NOTE making the slave move more than this could damage the releasing system,
the diaphragm spring in the PP is only made to move so far before the fingers will be permanently bent,
or the release arm bushing may be crushed, or the ball pivot broken off the bell housing.
NOTE Also the spring should not fully compress as the more it compresses the more stress the metal will see,.
NOTE eventually the spring will shatter , if its made to operate to full compression or coil bind or close to that point.
and then small shards of metal will be sucked into the seals of the MC bore then sent to the slave this will destroy the seals.
(I found 2 different cars with shattered springs .)
SO removal of 2 full circles of the coil spring should give the spring enough strength to push back.and not go into coil bind.
Also note the hydraulic force being put on the piston from the slave cylinder while its forcing the diaphram spring to open
NOTE make the end of the spring that you cut off as flat as possible so its not side loaded to drag on the MC bore
I don't know if anything will be damaged by more travel, but I do know that some cars do need more travel. On my '82, even with the correct MC the clutch would not quite fully disengage when first starting in cold weather. In warm weather it would just make it, but nothing to spare.
yes this trimming is for the dual disc system.
However if the system will not properly release single disc (then first verify you have the correct S4 slave cylinder )
as the 85/86 version can sometimes be found and this slave will not give enough release distance,
this will ruin your new clutch.
Also replace the flex line with the long flex version to allow for fully removing the air from the lines.
1) Getting the right master cylinder. Or is there only one manufacturer at this point?
2) trimming down the piston the right amount. 5mm, right?
3) trimming the spring down the right amount. I haven't seen anything specific on this.
1) Getting the right master cylinder. Or is there only one manufacturer at this point?
2) trimming down the piston the right amount. 5mm, right?
3) trimming the spring down the right amount. I haven't seen anything specific on this.
Yes, Yes, trim 2 coils off. See red mark in this pic
11-07-2013, 11:04 PM
