Blown Slave Cylinder

Neil

Supporter
When I was building my car I found a problem with the Porsche G50 clutch slave cylinder. It leaked when it was over half extended but not at rest. I dissasembled it and found that there was rust pitting on the bore but only in one area. The brake fluit had probably absorbed moisture and over time, rusted the bore. A new slave cylinder fixed the problem.
 
When you set it up, and you released the pedal, did the slave retract fully? Or did it only partially retract.

If only partially then the next full,press on the pedal will over extend it.

Perhaps a spring to ensure the slave retracts fully.

Ian
If the pedal goes all the way back would that indicate the slave is travelling all the way back?
 
If the pedal goes all the way back would that indicate the slave is travelling all the way back?
No, its like brake callipers, when the pedal is all the way back the pistons just loose the brakes.
The slave just sets at a point. When you have an adjustable rod you can set that rest point.
 
No, its like brake callipers, when the pedal is all the way back the pistons just loose the brakes.
The slave just sets at a point. When you have an adjustable rod you can set that rest point.
Is it the clutch fork that's pushing it back? If that's the case then the piston will get pushed back into the same spot it always has? I'm just finding it strange that the setup has been this way since 2005 and then all of a sudden it randomly blows seals. Hopefully I will get a chance to have a better look tomorrow though.
 
So the rod at the end of your slave will press against a lever / fork or something to make the clutch work.
Is there any way you can put a spring on that to ensure the slave cylinder returns fully from the extended position?
If it is not returning fully, the next time you press the pedal you will over extend the slave.

What are the diameters of your master and slave cylinders?
Rule of thumb is the slave should move 3/4 inch / 19mm with full depression of the pedal
Is the master too big so able to move more fluid to the slave than actually needed?

Ian
Can you clarify what you mean when you say over extend? As in it travels outside of the bore? Or as in (I will try my best to explain) if you push it and it moves 2cm then release it goes back 1 cm then press it again and then it attempts to push 2cm but it cant go that far and blows past the seal?

Being a hydraulic seal I thought they would be much stronger. It's not like I'm trying to push the pedal through the floor or anything.
 

Ian Clark

Supporter
Hi Mitchell,

When your looking at the clutch slave cylinder out of car, the piston is already fully extended, there is no more available to open the clutch.

If the clutch slave cylinder pushrod is not pushed back into its retracted position by the clutch fork you will have no clutch action when you push on the pedal.

Comes back to my asking you look at the location of the clutch fork. If it's moved that has to be fixed first. Have a look, I've been down this road already...

Cheers
Ian
 

Ian Anderson

Lifetime Supporter
Supporter
Can you clarify what you mean when you say over extend? As in it travels outside of the bore? Or as in (I will try my best to explain) if you push it and it moves 2cm then release it goes back 1 cm then press it again and then it attempts to push 2cm but it cant go that far and blows past the seal?

Being a hydraulic seal I thought they would be much stronger. It's not like I'm trying to push the pedal through the floor or anything.
that explains what I was trying to say

Ian
 
Ian, that pic looks like my C35 transaxle. That spring acts more as an anti rattle cause it does not have enough tension to return the slave.
 

Ian Anderson

Lifetime Supporter
Supporter
Ian, that pic looks like my C35 transaxle. That spring acts more as an anti rattle cause it does not have enough tension to return the slave.
It was the first picture that came up on a google search.
but how often have you seen that spring missing?

Ian
 

Bill Kearley

Supporter
MAKE SURE YOU START OFF WITH A QUALITY SLAVE CYLINDER .THE CLUTCH SPRINGS SHOULD RETURN THE SLAVE TO THE SAME RESTING SPOT EVRY TIME WHILE AT THE SAME TIME THE PEDDLE IS PUSHED BACK TO IT'S RESTING SPOT. INSTALL A PEDDLE STOP AND ADJUST TO ENSURE ENOUGH TRAVEL FOR DISENGAGMENT. ALLOW THE PEDAL TO REST ON THE PUSH ROD ( FLOAT ). INSTAL A VERY LIGHT SPRING ON THE CLUTCH SHAFT LEAVER JUST TO STOP A POSSIBLE RATTLE AND KEEP SLAVE PUSHROD ENGAGED WITH CLUTCH SHAFT. THIS METHOD WILL ENSURE YOU DON'T ALOW A PUMP UP STIUATION TO HAPPEN THUS OVER EXTENDING THE SLAVE AND POSSABLY PUSHING THE PISTON TO FAR.
 
If the master cylinder reservoir height is lower than the slave cylinder, it's (slightly) possible to bleed air back into the system.
 

Ian Clark

Supporter
All valid recommendations, especially given the many variables in hydraulic clutch slave actuation and transaxle types.

In the case of the OE Audi application we went though in my shop, the complete clutch assembly and slave (just to be safe, it wasn't leaking) were replacred. I was surprised to find there are no adjustments of any kind to the slave cylinder and clutch fork relationship.

The slave cylinder goes in to one depth and picks up the fork. The fork has no adjustment either so it is under constant light pressure from the spring inside the slave cylinder. Which means the throw out bearing is always running, although not with any significant load unless the clutch is being disengaged for a shift or held open while stopped with the transaxle in gear.

I suspect the answer for this situation will come down to the clutch fork/slave cylinder pushrod alignment. If not I'm always ready to learn something new:)

Cheers
Ian
 
All valid recommendations, especially given the many variables in hydraulic clutch slave actuation and transaxle types.

In the case of the OE Audi application we went though in my shop, the complete clutch assembly and slave (just to be safe, it wasn't leaking) were replacred. I was surprised to find there are no adjustments of any kind to the slave cylinder and clutch fork relationship.

The slave cylinder goes in to one depth and picks up the fork. The fork has no adjustment either so it is under constant light pressure from the spring inside the slave cylinder. Which means the throw out bearing is always running, although not with any significant load unless the clutch is being disengaged for a shift or held open while stopped with the transaxle in gear.

I suspect the answer for this situation will come down to the clutch fork/slave cylinder pushrod alignment. If not I'm always ready to learn something new:)

Cheers
Ian
I would imagine the alignment shouldn't change as it was fine before and the slave cylinder slides into the hole so there is no alignment option. Unless it's either always been like that or something has happened?

The only other issue I had was the starter pinion disappeared at the same time as when the first seal broke which I imagine was due to the car starting with clutch pedal in but clutch was still engaged due to leaky slave. Will see if I can figure anything out today.
 

Ian Clark

Supporter
HI Mitchell,

Lots of things happen you wouldn't imagine, until it does! If you look strait into the hole the clutch slave cylinder presses into, you should be able to see the end of the clutch fork and the small pocket the pin of the slave cylinder seats into centered in your line of sight.

If it's there, I'm out of ideas... if it's not the transaxle will most likely have to come out (unless you can somehow fiddle the fork back into the sightline). Even then it would be good to know everything is ok inside the bellhousing, that might also shed some light of your starter pinion issue.

Cheers
Ian
 
HI Mitchell,

Lots of things happen you wouldn't imagine, until it does! If you look strait into the hole the clutch slave cylinder presses into, you should be able to see the end of the clutch fork and the small pocket the pin of the slave cylinder seats into centered in your line of sight.

If it's there, I'm out of ideas... if it's not the transaxle will most likely have to come out (unless you can somehow fiddle the fork back into the sightline). Even then it would be good to know everything is ok inside the bellhousing, that might also shed some light of your starter pinion issue.

Cheers
Ian
The starter pinion has been fixed, Im pretty sure it was only an issue as I had it in first gear when I started it and just had the clutch in. But as there was no clutch it just ripped the pinion apart. I had a look and found the hole to look pretty centered. The only thing I can think of is if the clutch fork is seized up for some reason and the pressure has nowhere to go so it goes past the seal? That or a fault in the slaves. I have ordered one from somewhere else so hopefully that fixes the problem.
 
so in regards to alignment, it basically gives no room for error. it slides into a hole with a perfect fit and is push in to a point were a bolt goes through transaxle to hold it in position. there is no adjustment for back and forth. as i put the slave in the piston is pushed back into bore until the slave is in the mounting position. then i start pumping the pedal to start bleeding it. then it starts leaking. i have my wife doing the pedal work but she has said that she has not been pressing hard on the pedal either, and even if she did i would assume it would require a decent amount of strength to push the the seals? As above, i have a new one coming from a seperate manufacturer so i am hoping that that shall fix it. otherwise i may have to pull transaxle out and see what the heck is going on.
 

Ian Clark

Supporter
Hi Mitchell,

Compare the slave cylinder length in your picture to the length measured from the mounting hole to where the slaves' pushrod would contact the clutch fork inside the transaxle.

The distance measured inside the transaxle should shorter than the bare cylinder by almost the length of the stroke of the pushrod. If that works out ok then either the fork is jammed or the two new slave cylinders are both bad, or something else is going on (throw out bearing stuck?).

If you wanted to check the slave cylinder outside the car to see exactly where its' leaking you would have to compress the pushrod in almost the full stroke, then try bleeding the part. It wont be easy to clamp or retain the pushrod in the retracted position but nothing will be learned by testing it without the pushrod retracted.

The chances of buying two bad slave cylinders is pretty low, however having three cylinders acting the same way would point to something other than the slave cylinders in my mind.

If the fork distance to the mounting hole dimension is the same or greater as your slave cylinder at rest the it's definitely the fork.

It's a great mystery at this point...

Cheers
Ian
 

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Ian Anderson

Lifetime Supporter
Supporter
Your clutch fork, part 11 in the above diagram, what is it like? There were reports on here that some are folded steeel but those can bend and allow slave movement but no clutch movement.

ian
 
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