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PeterAllen

Leaking clutch m/c - why is it so?

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If I am correct there is a member here with an LS1/t56 installed in their S30 and making enquiries on HybridZ regarding clutch master cylinders suitable for the t56. Apparently the Holden m/c installed in his vehicle is leaking and a HybridZ member has suffered the same fate – twice! As someone who is doing this install I am trying to figure out ‘why is it so’.

I thought that perhaps the piston is being forced too far into the cylinder and once the clutch was engaged the pressure was increasing the leaking back past the seal in the m/c, however, as both master and slave cylinders come from the same donor car this seems odd.

As illustrated in the photo, while the load distances are different they should, if anything, mean more pedal travel is required in the Datsun configuration to move the piston sufficient to engage the clutch and therefore unlikely to create over-stroking of the m/c piston. I am assuming that the plunger rod is aligned to the centerline of the m/c.

Does anyone have any theories on these failures or are they a statistical aberration… Why is it so?

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The distance the piston travels in the m/cylinder is important Pete...if you use a different m/c it must work exactly the same as it worked in the original car it was designed for

 

Not only should the piston travel be the same distance, it also should travel back and forth in the same position within the cylinder...deviate from this and troubles will ensue

 

The amount of pressure applied is not really an issue when everything is working ok unless there's abnormal resistance somewhere in the system, like a dry thrust-race support sleeve or a strong pressure plate...then there's blocked fluid holes and/or hoses

 

Usually when the clutch pedal is pressed in a stock vehicle, it's pushed to the floor and all the pressures, travels, alignments etc work properly for that car...swap them to a strange car, then they're gunna have to work the same, otherwise expect trouble

 

So the pedal must have the same stroke as the donor and the pedal/piston rod must be aligned true to the axis of the m/cylinder

 

The m/c piston must move back and forth in the same POSITION in the cylinder as before when it was in the donor

 

The pressure plate fingers must have ample travel to match the original donor movement of the MASTER cylinder piston and the slave cylinder piston travel....and the disengagement of the clutch plate must work WITHIN the ORIGINAL DONOR BOUNDS and TRAVEL of the master cylinder piston

 

Think more about the donor specs than the Zed when installing strange bits

 

Enjoy, Pete

 

 

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Pete - I agree with your synopsis and that’s why I opted to go with the donor car m/c, even to the point of moving everything 10mm to re-establish the donor car pedal ratio. However, the experience of others tends to suggest there is something flawed with the concept. It’s hard to fathom, if anything, incompatibility of the m/c and s/c would impact on the functionality of the clutch itself rather than a mechanical failure of a cylinder component.

 

Am I correct in assuming that in free load the piston would retract to a stop and there should be some (but minimal) play in the plunger rod, thus allowing the piston to always fully retract. I cannot imagine that both cars were not setup like this.

 

One point you raised is interesting – I wonder whether both users are running extra heavy duty clutches which required greater line pressure to actuate and this is resulting in seal failure. For this to be so the design must be operating at a very fine tolerance and one would think it would be a very common occurrence in t56 fitted Commodores generally.

 

Also, I understand that a replacement Holden m/c is around $200+ which might suggest there is something special about them, or am I just out-of-date with the prices of parts!

 

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Yes there must be a small amount of free play between the piston and pedal rod Pete

 

As far as leakage is concerned, possibly they were installing second hand m/cylinders do you think ? and not set up right...they could leak due to wear, then on top of that being asked to do a different clutch job to it's original intention ie working the clutch in a strange car...if they are using new m/c s' then yes the strong pressure plates could be causing excessive pressure on the piston seals in both m/c and s/c

 

There is also the length of the thrust race fork, that is the ratio from the pivot to each end...check that out, it's important

 

Then there's the pedal position in relation to the time when the clutch plate is released, the instant it is released that is....this may not sound relevant but the m/c piston could be in the wrong position when the clutch plate just becomes free of the flywheel and p/plate

 

But if the leak is inside the m/c itself, how do they know it's leaking past the seals ? if the piston travels too far or is in the wrong position at any stage of it's movement according to where it should be, fluid can't flow down from the reservoir ports or flows into the wrong place...so it's important that the seals can pick up fresh fluid each time the pedal is pressed...the seals must be behind the ports when pedal is up...that's why there should be free play at plunger rod

 

When the pedal is pushed, the piston seals must then pick up fresh fluid and the seal must travel past the port...sealing the fluid in front of the seals which in turn applies the pressure needed to work the s/c

 

Pete

 

 

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I might be on to something... I just had a closer look at the m/c and I've noticed that the fluid line from the reservoir has a swivel connector where it joins the m/c fitting. If the other owners have experienced fluid leaking in the engine bay then this could (must!) be the problem. You can also see that the m/c protrudes some 50mm or so through the firewall into the cabin. If the leak is occurring in the cabin then it is an issue with the seals and I'm back to square one. I await to hear from them.

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well im using a 7/8 pbr trailer master cylinder cheap to buy bolts into std holes and uses standard rod works fine and im running a textralia high pressure button clutch and have to say its one of the softest clutches ive driven

biggest trick ive found was making a flex tube from the bleed valve on the slave cylinder extended the bleed nipple up to the manifold level

made it so easy to bleed when i put gearbox in this time

 

must say i was concerned about swivel end on the push cylinder but has not leaked since i fitted it

going by geniune holden prices would say 200 was cheap was billed $55 for front crank shaft seal and that was trade price

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Hey Peter A.

 

I'm about to start on an engine conversion that will cause grief with the clutch master cyl interfering with the head/rocker cover if I have the engine positioned as far back as possible so I'm looking for the shortest possible MC. Can you confirm the total distance the Commodore MC protrudes into the engine bay? Also the bore size?

 

Anyone got any other suggestions for clutch MC's that protrude the least amount into the engine bay?

 

Even with the shortest MC I may be faced with having to engineer an underdash alternative.

 

Thanks.

 

David

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... Can you confirm the total distance the Commodore MC protrudes into the engine bay? Also the bore size?...

 

Sorry, I'm not keen to pull it apart just yet (I already have a shed full of such items!) so I don't have the bore size, but the engine bay dimension is 70mm.

 

Not much room left when you fit a V12!!!

 

(edit) Curiosity got the better of me - the bore is 3/4"

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Hey Pete I'm back. If theres any dimensions you need let me know. Otherwise I have noticed that this master doesn't use the standard hole in the clutch pedal, there is one drilled higher up meaning less travel than standard. I was thinking about stealing a 240/280z MS to try on my car just to see if that works.

 

Interesting about that trailer MC, cheap price and it works.

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rb30x - Okay, we might be on to something. I take it the m/c in your car is mounted higher than the original m/c - I presume because of interference with the rocker cover or head, please confirm. I also take it the push rod is therefore not aligned to the centreline of the m/c - or is it? Has the pivot been relocated higher on the pedal to compensate?

 

Could you also confirm that the leak is from the bore inside the car and not the swivel union as shown in the earlier post. Thanks.

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Thanks for the bore size Peter.

 

Unfortunately 70mm is likely to be too much so I think an underdash unit will be the only way to go.

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It leaks from the boot. It also looks as though it has been mounted slightly higher but still in the same hole in the firewall. You can see the original hole for the piston rod.

 

Picture250636833.jpg

 

Picture250636832.jpg

 

Picture250636834.jpg

 

 

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rb30x, The push rod looks nice and straight so it can't be pushing the piston off to one side..which can cause leakage past the seal or seals..how many seals does that m/c have on the piston ?

 

The leakage out the boot could be natural seepage you get in any cylinder...are you constantly topping up the reservoir ?

 

 

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The leakage out the boot could be natural seepage you get in any cylinder...

 

I dont know about that..Ive never seen it in my time

 

Im assuming its a secondhand setup from a donor car?

 

Id be pulling it out. clean, hone and rekit at the very least.

 

Put an adjustable rod in it and fine tune as you go

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...how many seals does that m/c have on the piston ?...

Two... There was quite a bit of crud in there for something that was only 2.5 years on the road. Also, the casing of the m/c appears to a lightweight moulded material, all in all not conducive to longevity, however, I haven't heard anything about it being an issue with Holdens.

post-684-144023571459_thumb.jpg

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Two... There was quite a bit of crud in there for something that was only 2.5 years on the road. Also, the casing of the m/c appears to a lightweight moulded material, all in all not conducive to longevity, however, I haven't heard anything about it being an issue with Holdens.

 

 

I'm ashamed of you Pete, putting that toy of a thing in your Zed...is it going in your car ? I hope not

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Well after looking at that one mine does look brand new. I might strip it and clean it again, just incase.

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