Duratec R400 clutch slave cylinder

[John Vine]

This is a follow-up to my earlier post.

I've yet to receive the definitive diagnosis from the garage, but is it possible for the clutch slave cylinder mechanism to become "over-extended" in any way?  In other words, could it "pop out" as it were -- after, for example, too great a piston stroke at the master cylinder?  And if so, would it then remain in this extended position, partially disengaging the clutch, even when the pedal is at rest at the top of its stroke?

If so, it would fit with the symptoms I experienced:

• Immediate and severe clutch slip, with almost total loss of drive
• Loss of all free-play in the pedal
• No clutch engagement at all, other than at the very top of the pedal travel

I'm beginning to wonder: did I set up something incorrectly when I replaced the master cylinder?

Of course, the problem could well be in the clutch itself -- time will tell.

JV

[ChrisC]

I guess it would be easy to test the master cylinder, disconnect it and press the pedal, but you would have to blead it again and clean up the mess.

 

[7 wonders of the world]

John I understand it there are different thicknesses of alloy spacer available that sits between the concentric slave cylinder (CSC)  and the bellhousing, might be worth a call to CC parts to get a dimension or drop mic on here a mail. 

[7 wonders of the world]

I have one here I will try to find it and measure the thickness, happy to post it too you and replace it later should you need it.

[John Vine]

As far as I can tell, the (new) clutch master cylinder appears to work just fine, and has done for several hundred miles following installation.

Would it be possible, on pressing the clutch pedal, for it to operate (expand) the slave bellows as normal, but then not to draw the fluid back again once the pedal was released?  In other words, could it operate as a one-way valve, pressurizing the slave but not subsequently releasing the pressure? 

This seems very unlikely to me as the pedal operation (pressure, feel etc) seems entirely normal, apart from the loss of free play at the top of the stroke.  I'm assuming this pedal pressure is simply the reaction of the diaphragm springs in the clutch cover.

The symptoms seem to suggest that the slave bellows are not returning to their normal rest position.  Is this possible?

JV

[John Vine]

Thanks for that, 7WotW.  As my earlier thread explained, the car is sitting in Marcassus Sport (Toulouse) awaiting delivery of the full set of clutch spares (slave, clutch cover, clutch disk).  When they arrive (next week, I hope), the engine will come out.  Until then, we won't know what the precise problem is.

Could the spacer you mention become loose or detached in any way?  Presumably, as the clutch has worked just fine for 35K miles up to this point, the spacer would already be the correct one?

JV

[mic]

The slave cylinder can be over extented, either by continual pumping of the pedal or having the pedal stop incorrectly adjusted. The spacers are available in different thickness, but unless the wrong slave has been fitted the one already in the bellhousing should be OK.

[John Vine]

Thanks for that, mic.  I don't think I'd been pumping the pedal.  As I recall, I was changing up when suddenly the pedal seemed to "firm up" and all the usual free play (the slack before the clutch begins to disengage) had disappeared.  Even the slightest pressure on the pedal at that point would disengage the clutch fully.  The position of the pedal stop hasn't been altered since I assembled the kit in 2008.

What puzzles me is how the slave bellows could extend too much when the CRB at its end is being pressed firmly against (and being resisted by) the diaphragm springs.  Would there still be sufficient lateral movement of the bellows for the over-extension to occur?

If the culprit is indeed the slave, I need to understand how the problem could have arisen if I'm to have a sensible discussion with the garage!

Btw, when I bled the new master cylinder (with my wife's assistance), we were very careful to press the pedal very smoothly and gently. (But surely the bellows would barely move during bleeding, there being almost no hydraulic pressure?)

JV

[John Vine]

I've just come across this guidance for fitting concentric slave cylinders.  It makes interesting reading and supports mic's comments.  It also says: THE ENTIRE CLUTCH MECHANISM SHOULD BE CHANGED WHEN FITTING A
NEW CONCENTRIC SLAVE CYLINDER.  That's what Marcassus are proposing to do, so I guess I should be reassured by that!.

JV

[7 wonders of the world]

So seems the use of CSC's is not a signifiacnat step forward.............

The over extension must point to the inability of the CSC to return swiftly to its rest position, either insufficient return pressure or very small internal fluid ports?

Wonder if the SBD version suffers from this ?

 

Is a datum set point for the clutch pedal stop from CC?

Likewise I'm guessing CC must be able to provide data for the spacer ring thickness as the variation in these must be to compensate for difference in bellhousing / block face to CSC / bellhousing.

[Rob Walker]

To follow on from Mics post, it has been my experience that the slave cylinder fitted without restriction can lead to the clutch cover fingures being damaged by over extension, this then allows the slave cylinder to over extend.  I always fitted a stop on the master cylinder actuator rod set up to the point at which the clutch disengauged.  This was a 50mm length of flat bar with three holes in it the centre one the slave cylinder actuator rod passed through the other two holes had screwed rod and lock nuts  on them so that the bar could be mounted to the peddle box and be adjusted in or out to restrict travel in the actuator rod.  Sorry I no longer have a Duratec car to photograph for you.

Rob

[John Vine]

Assuming the slave is the culprit, I'm beginning to think that I may have made some sort of error setting up the new master cylinder.  It seems too much of a coincidence that I've encountered these problems within 300 miles or so.

When I fitted the new m/c (which appeared to be identical in every way to the old one, although I didn't get it from CC), I was puzzled that, to get the clutch to disengage fully, I had to lengthen the piston rod by about 1cm compared to the old setting.  With the rod set in the same position as the old one, I simply was unable to select a gear at all.  I re-bled the system several times, but with the same result. 

However, I'm certain that there was still some free play in the rod -- there was certainly some in the pedal action.  Without free play at that point, of course, I suspect the release valve (#1 below), which allows fluid to return freely to the reservoir, might not open properly, in which case it seems possible that the slave could end up over-pressurized.

 

Could clutch riding have the same effect?  I admit to doing this from time to time, albeit very lightly, when my left ankle gets tired.

The other possibility is that the slave itself had become defective in some way and was unable to return fully to its rest position. 

I've asked CC if they can shed any light on what may have happened.

Regarding the pedal stop, I set this at build time, and haven't adjusted it since.  I followed the Assembly Guide, but it didn't give any specific measurements, settings or datum point.

No doubt things will become crystal clear when the engine comes out....

JV
 

[John Vine]

I always fitted a stop on the master cylinder actuator rod set up to the point at which the clutch disengauged.

Would the standard pedal stop (as fitted to the front of the footwell) achieve the same end?

JV

[Rob Walker]

Yes

[John Vine]

Thanks, Rob.

JV

[John Vine]

See the update at the end of my earlier thread.

JV