Detectives Needed - Clutch Mystery !

Peter Delaney

GT40s Supporter
Here's a little mystery to start off 2010 :

About 6 months ago, DRB#27 developed an interesting problem with the clutch (SB mated to G50 with spring-style Porsche clutch plate). When going down a hill & stopping at a stop sign, I change down thru the gears to 3rd or 2nd, then at about 10kmph, clutch in & go to neutral, then brake to a stop.

OK so far, BUT, when I depress the clutch, I find that it has gone quite "soft" - almost no resistance until halfway in, then some pressure. I don't think that it has fully released as it is a bit hard to select 1st in that state.

One pump on the pedal, and all is back to normal again !

I have bled the system, checked for leaks, ensured that the fluid level in the m/c is correct - yet I always get the same problem under the same circumstances (but never under any other driving conditions).

As mentioned, this problem surfaced about 6 months ago. That was around 6 -8 months after a new clutch plate was fitted, but nothing at all was touched around the time that the problem appeared.

So its a complete mystery !

Any detectives out there willing to toss some ideas about ?

Thanks & Kind Regards,

Peter D.
 
My guess is that there is dirt (or a bad seal) in the master cylinder allowing the pressure-side seal to leak on occasion.
 
Peter

I think I would also suspect the clutch master cylinder seal, a new set is not an expensive item and will be more supple. Seals can harden with age and function OK on a couple of quick pumps but miss seal on the initial application.

Steve
 
Hi Steve and a Happy New Year.
I had a similar problem when trying my clutch out. It was simply air in the system even after I was convinced I had bled it correctly. I had to remove the cylinder and bleed it with a friends help. Make sure the cylinder is pushed all the way in and the bleed nipple is pointing directly up. I think mine had a little air trapped in it, but once removed the pedal felt completely different.

Regards Martin.
 
If the problem is occasional (that is, it comes and goes), it's likely a bad cylinder--either master or slave. I've experienced this numerous times in Panteras, with exactly the same symptoms you describe, and it was almost always (but not exclusively) the master that had gone bad. While it certainly can't hurt to re-bleed the system one more time, it's probably time to refresh your hydraulics.

Depending on which setup you have, it may be cost-effective to rebuild vs. replace. On the other hand, if it's a real PIA to change the components out, as in a Pantera, then you're probably better off just replacing with new.

As an aside, such failures are usually attributed to failing to cycle the system periodically. I have had hydraulic clutch-equipped cars laid up for years at a time due to unrelated mechanical problems; I always made a point of trying to cycle the clutch through its full travel several times, at least once a month. If the car has been standing for some time, it's best to make your initial clutch application very slowly, as it's possible for the seals to stick. Moving them slowly and gently allows clutch fluid to lubricate them before they simply tear.

Good luck!
 
Peter,
You mention it only happens when going down hill & after making the 3 or 4 gearchanges/clutch applications. If you simply use the brakes & only shift from 4th/5th to neutral or first to get underway again does it do the same thing? It might be a combination of the reservoir height, connections on hose fittings to this & the condition of the cups/seals in the master cyl & frequent clutch operation while in the downhill attitude.
 
Pete, perhaps try a vacuum bleeder, or a pressure bleeder if you haven't yet (assuming you're using a simple pump and close valve or speed bleeder valve bleeding system...). And, perhaps try bleeding with the car on a slope, with the end being bled up-slope.
 

Russ Noble

GT40s Supporter
Lifetime Supporter
Peter,

I'd be picking a lazy seal in the slave which finds it easier to draw air in on release of the pedal rather than drag fluid back from the lowered mastercyl. On the flat the increased head of fluid is enough to prevent that. Probably just insufficient wall tension on the seal. It'll get worse with time. A new slave seal which should cost less than a cup of coffee will more than likely cure this problem.

I had a vehicle a few years ago that kept sucking air past the slave seal, the cylinder was 'as new' and when it did it for the third time in just a few months after having a new seal each time, I decided to machine an extra groove in the piston and fitted an extra seal the other way round. That was the end of the problem. The extra seal also acted as a 'wiper' and kept any detritus that got past the boot, away from the primary seal.

In your case though, provided the cylinder itself is OK, simply renewing the seal should be all that's required.
 
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Russ,

Sorry, but I'm not seeing your scenario. The clutch slave piston is not sucked in by the release of the pedal in normal circumstances. The pressure plate (and linkage) pushes the piston back against the brake pedal resistance. There is always positive pressure in the system until the pedal is completely released.
What may have been happening in your case was that the (virtual) height of the reservoir was below the height of the slave cylinder (under braking, without the clutch being used) and allowing a bad seal to leak air back into the system.

But, in Peter D's case, I don't think that air would spontaneously and completely leave a system that leaked air, without seeing fluid leak from the slave under rest conditions.
 
Hi Peter,
check the breather/air inlet in the res cap, a blockage or misplaced seal here could cause a slight vacumm which on release of the master cylinder could draw the slave piston further down the cylinder and giving excess clearance until you press the pedal twice,
also check the end float of the crank, with the motor being downhill any excess end float will take the clutch further away from the release bearing this is then taken up when you press the pedal the first time.
Not a common cause ,but I experianced this with one of my racing Jaguars in the seventies (engine just about worn out!)
hope you get it sorted without major expense,
cheers
John.
 

Randy V

Staff member
Admin
Lifetime Supporter
I can't help but think that the problem is due to the level of the MC being below that of the slave cylinder..
In the world of hydraulic brakes, we use residual pressure valves to ward off the idiosyncrasies of systems like this.. Not sure how we could address it for a clutch unless you could use a 2# residual valve in that line...
 
I don´t think it is the level of the master cylinders which defines the need for an residual valve. It moreover the level of the fluid reservoir compared to the level of all other componenents.
As long the reservoir is higher than anything else there should be no emptiing back into it.
TOM
 

Russ Noble

GT40s Supporter
Lifetime Supporter
Russ,

Sorry, but I'm not seeing your scenario. The clutch slave piston is not sucked in by the release of the pedal in normal circumstances. The pressure plate (and linkage) pushes the piston back against the brake pedal resistance. There is always positive pressure in the system until the pedal is completely released.
Bob, I agree with your theory, but experience beats theory everytime!

What may have been happening in your case was that the (virtual) height of the reservoir was below the height of the slave cylinder (under braking, without the clutch being used) and allowing a bad seal to leak air back into the system.

But, in Peter D's case, I don't think that air would spontaneously and completely leave a system that leaked air, without seeing fluid leak from the slave under rest conditions.
The vehicle in the example was my first Rangerover, the cause was insufficient wall tension on the new seals, similar to what you will get with worn/aged seals. During my years of hands on experience in the vehicle repair business I have struck air coming back through the slave seal several times, this is on 'normal' road cars and often there is no sign of fluid leakage. It is not a particularly common fault, but it does happen.

Peter's scenario would suggest he is experiencing the onset of this particular problem.

At the end of the day we can discuss these things til we are blue in the face, but the easiest and quickest way is to remove and inspect the cylinders, connections and seals, and renew as required. Preferably one at a time, so we can determine the cause. If this doesn't effect a cure then we have to start looking for some obscure problem, but since the system was working fine until a few months ago it's most likely to be tired seals or corroded or scored cylinder bores.

Easily, quickly and relatively cheaply fixed. Normally I would look at the master cylinder first and if I could find nothing wrong then I would do the slave, but Peter was looking for ideas on what it could be, lazy slave seals is my pick.

The other alternative is that under downhill braking, which may be more severe and continuous than normal driving, and depending on orientation of the slave and the clutch arm, the clutch arm could be pushing the slave piston back, thus causing the long and light pedal travel after coming to a stop. But why it should start doing it now would be a mystery..... However a temporary helper spring on there to hold the arm and release bearing against the plate (ie away from the piston) would easily confirm or eliminate that scenario.

Actually, if your cylinder/arm does fit that scenario, that would then be my first pick.

In years of repairing mainly manual trans cars, the number of times I have struck air coming in from the slave can be counted on the fingers of one hand so make that my number two pick!
 
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Brian Magee

Supporter
Being a real pesimist here, years ago I had a Triumph 2500. The crankshaft thrust bearing had worn so much that when you pressed the clutch the first part of travel was pushing the crank forward.You then had to pump the pedal a couple of times to get enough movement to free the clutch. As the thrust bearing wore eventually that did not free the clutch and the engine had to come out. An easy one to eliminate from your search, get someone to see if the front pulley moves forward when you press the clutch.

Brian.
 

flatchat(Chris)

Supporter
You could be onto something there Brian , --the G50 has a pull clutch -- so when going down hill the crank would slide forward and require a couple extra pumps to pull it back again Hmmm , Uh Oh ! --thats a energine problem:cry:
Also check that the clutch fork pivot shaft is in securely
 

Peter Delaney

GT40s Supporter
Wow - thanks for all your ideas guys !

In spite of many experiences with this car, I am going to start out as an optimist !

The first thing I'll do is cross my fingers & check to see if there is any fore & aft movement of the harmonic balancer / pulley whist operating the clutch.

If (hopefully) nothing is found amiss here, I'll get the m/c reconditioned (or buy a new one, depending on price). If that fixes the problem, I'll still do the same thing with the slave cyl - simply as a precaution.

Now that I think about it, these parts went into the car some 10 years ago, & sat inactive for the 4 year build. After that, she has only done 12,000kms, but seldom more that one drive per week. So a complete refresh of the hydraulics would be a good thing regardless.

I'll post the results of each phase here as I go.

Thanks again - much appreciated.

Kind Regards,

Peter D.
 

Peter Delaney

GT40s Supporter
Hmmm - been thinking about the push/pull clutch issue :

I can understand Brian's situation with a push clutch - if the crank thrust bearing wears & the crank moves forward, it makes sense that the clutch mechanism has to provide a longer throw to disengage (assuming that the clutch release bearing "floats" back in its normal position) - hence the extra pumps of the pedal to extend the travel. (However, if the crank was able to move that much, you would have to figure that some serious damage had already occured to the bearings at both end of the rods due to mis-alignment).

BUT, in the case of a pull clutch, any forward play in the crank would pull the inner ears of the clutch fork forward, thus pushing the slave/cyl end back & hence the pushrod further back into the slave/cyl. Therefore the longer stroke of the slave/cyl should compensate for the forward play in the crank (or not fully release the clutch if the supply of fluid flow runs out) ?

Regardless of the above, this got me thinking about the push vs pull - am I right in thinking that the standard Ford SB setup uses a push clutch ? If so, are we tempting fate by fitting a pull clutch (which tries to drag the crank back) to a motor whose crank thrust bearing setup is maybe only designed to resist forward movement courtesy of a push clutch ?

Any further thoughts ?

Thanks & Kind Regards,

Peter D.
 
Bob, I agree with your theory, but experience beats theory everytime!
I have almost 50 years experience in this kind of thing (including 15 years as a mechanic and the balance keeping 1000 customer's ERAs on the road), but I've never seen a problem as you exactly describe it. There may be factors that aren't obvious, but the probability of a bad master in the original (not your) circumstance is greater than 90%, IMHO...
 
Hi Peter
The crankshaft thrust bearings in the SBF are integeral with the centre main bearing and is therefore more stable in its location than the old style two piece C shaped thrust bearing used in the old Jaguars (my post 10 and Triumphs Brians post 14), when this type of bearing wore thin they tended to fall out of there housings and give massive and noisy crank endfloat,thankfully not an issue with the combined mains in the SBF.
I do not think that the SBF design favours either push or pull on the crank,long periods of holding the clutch down will put a bit more wear on the thrust (crankshaft) bearings but usually after very high mileages.

The end float should be 0.004-0.008 when building up a new or re-con motor, this often increases to around 0.010- 0.012 in service.
Given that your instalation is about ten years old, I would service/replace the slave cylinder especially if its close to the exhausts as on UN1 equiped 40s
cheers
John
 
While doing some other research on a G50 site the other day I noticed that they occasionally have problems with the lugs that the release bearing cross shaft runs in failing or distorting, I assume that due to the 'pull' type actuation these simply try to lift or pull away from the casting, just another possibility to investigate if all other remedys fail.
 
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