Leaking clutch m/c - why is it so?
Posted 15 May 2010 - 12:50 AM
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?
Posted 15 May 2010 - 10:45 AM
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
Posted 15 May 2010 - 12:57 PM
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!
Posted 15 May 2010 - 07:57 PM
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
Posted 15 May 2010 - 08:56 PM
Posted 15 May 2010 - 09:50 PM
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
Posted 16 May 2010 - 05:45 PM
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.
Posted 17 May 2010 - 07:33 AM
... 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"
Posted 17 May 2010 - 08:46 AM
Interesting about that trailer MC, cheap price and it works.
Posted 17 May 2010 - 11:35 AM
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.
Posted 17 May 2010 - 12:59 PM
Unfortunately 70mm is likely to be too much so I think an underdash unit will be the only way to go.
Posted 17 May 2010 - 02:58 PM
Posted 17 May 2010 - 05:10 PM
The leakage out the boot could be natural seepage you get in any cylinder...are you constantly topping up the reservoir ?
Posted 17 May 2010 - 05:51 PM
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
Posted 17 May 2010 - 06:58 PM
Posted 17 May 2010 - 10:03 PM
Posted 17 May 2010 - 10:08 PM
Posted 17 May 2010 - 10:12 PM
Posted 17 May 2010 - 10:27 PM
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.
...how many seals does that m/c have on the piston ?...
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