Blown Slave Cylinder

[Mitchell B]

Hey guys and gal's,

I have recently replaced by slave cylinder in the transaxle. It was a brand new unit, however it was not bleeding correctly as well and upon further investigation realised it was spitting fluid out into the rubber boot. This is the 2nd brand new unit I've put in and it has blown both of them. Am I just unlucky or is there something I'm doing wrong?

I sent almost a whole bottle through the line with no slave attached first first make sure there is fresh fluid the whole way through, then just attempted to bleed it as normal. Pump and release with bleed screw.

I'm at a loss and it's starting to become very expensive.

Thanks

M

 

[Randy V]

Slave cylinders should have adjustable “physical” stops to eliminate this happening. I have seen this also applied on the master cylinder input side. Else, you can just force the slave cylinder piston out of its bore as yours may have done. This damages the piston, the cylinder and of course the seals.

 

[DaveM]

What is the brand of the slave cylinder? From your description it's an internal concentric type?

 

[Mitchell B]

Hey Dave,
It's ACS branded and I've attached a photo below.

What is the brand of the slave cylinder? From your description it's an internal concentric type?

 

[Mitchell B]

Slave cylinders should have adjustable “physical” stops to eliminate this happening. I have seen this also applied on the master cylinder input side. Else, you can just force the slave cylinder piston out of its bore as yours may have done. This damages the piston, the cylinder and of course the seals.

Hey Randy, it does have a physical stop. The piston can not come out of the bore. I will try add a photo. It's got an internal circlip style setup.

 

[Mitchell B]

Looks like I'm using the wrong names for the posts qs well. Apologies.

 

[Randy V]

Hey Randy, it does have a physical stop. The piston can not come out of the bore. I will try add a photo. It's got an internal circlip style setup. View attachment 118191

If it is leaking fluid from the boot, the seal is damaged and the slave will need to be rebuilt or replaced. It may be possible that it was damaged at the factory during assembly.
If fluid is leaking out, it is being replaced with the arch enemy of any hydraulic system - air...

 

[Mitchell B]

The only problem is its the 2nd brand new one I've out in and had the same thing happen. Is there anything that would cause them to blow the seals straight away? As in if there was air in the system would it blow the seal out? Or could there be something wrong with the clutch for example of the piece the slave presses against was locked up and stuck in position would the pressure just have to go somewhere and blow the seal or would you just not be able to press the pedal? I just want to make sure I'm not doing something stupid to be causing it, before I go and try to claim a faulty part with the company.

 

[Randy V]

The only problem is its the 2nd brand new one I've out in and had the same thing happen. Is there anything that would cause them to blow the seals straight away? As in if there was air in the system would it blow the seal out? Or could there be something wrong with the clutch for example of the piece the slave presses against was locked up and stuck in position would the pressure just have to go somewhere and blow the seal or would you just not be able to press the pedal? I just want to make sure I'm not doing something stupid to be causing it, before I go and try to claim a faulty part with the company.

It sounds like your slave may be at full travel when your pedal still has room to move further. Stepping harder on the pedal, just builds up pressure in the slave (it is already at the snap-ring limit) and now you are building pressure beyond the limits of the slave. This can blow seals and I have seen it actually crack the slave casting.
Something is out of adjustment or out of spec here.
You need to insure that the pedal motion is not so great as to try and overcome the physical limits of your slave cylinder.
Perhaps if you shorten the pin from the pedal arm to the master, it would allow the pedal to be fully depressed just at the limit or just prior to the limit of the slave?

 

[JP Verweij]

It sounds like your slave may be at full travel when your pedal still has room to move further.

Thats what I think also. When I calculated my slave, I made sure it had enough capacity that it would handle the stroke of the master.

 

[Mitchell B]

I guess that may be an option and sounds like a potential cause however nothing should have changed in the car (unless something has broken). Will shorten the master cylinder arm and keep adjusting I guess to ensure clutch works properly. Just seems strange that the car was running fine and then all of a sudden its blown the seals.

Thanks for your help!

 

[Mitchell B]

Further to above, I get setting it up for the first time there would be an issue. The other symptoms it had was when I removed it completely and pushed the pedal down it built up more pressure and only had a small leak when it was compressed against the retaining ring. So it's only when the piston is further down the barrel that it spews out a substantial amount.

 

[Ian Anderson]

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

 

[Mitchell B]

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

Hey Ian,

The pedal retracts every time with no issue returning to the unpressed position. It's impossible to tell from what i can see to which position the slave is in and whether it is getting pushed all the way back. (Unless I'm misunderstanding what your asking) When I install the slave I can feel the piston getting compressed as it has no fluid in it at that time.
Being inside the bell housing does not help.

 

[Ian Anderson]

Hey Ian,

The pedal retracts every time with no issue returning to the unpressed position. It's impossible to tell from what i can see to which position the slave is in and whether it is getting pushed all the way back. (Unless I'm misunderstanding what your asking) When I install the slave I can feel the piston getting compressed as it has no fluid in it at that time.
Being inside the bell housing does not help.

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

 

[Terry Oxandale]

I would think if the seals are good, then even hitting the stop-ring shouldn't cause a leak. If that was the case we'd have leaking brakes all the time. Are the cylinders "new", or remanufactured? I know in the past when I've had to use something other than a factory MC, leaking was almost expected. Just for clarity, the leak is on the line end of the MC (front seal), and not on the pedal end of it (back seal)?

My curiosity would have gotten the better of me at this point, and I would have removed the stop-ring, pushed the piston out, and taken a gander at the seals (installed backwards, rolled, torn, etc), as well as the bore quality (pitted, scored, etc). Would look to ensure the seal is fully in front of the inlet port so that there isn't some kind of bypassing of the seal when you initially depress the pedal (bleeding difficulty/leaking).

 

[Ian Clark]

Hi Mitchell

Getrag Transaxle? Is it possible the clutch fork inside the bellhousing has become misaligned?

If the clutch slave cylinder pushrod doesn't engage the clutch fork there is nothing to hold the clutch piston in its proper resting position to start the action of pushing against the fork to lift the clutch pressure plate when you push on the clutch pedal.

Probably won't bleed properly either and the seals may be resting with no pressure on the lip of the seal...

It's a weird one for sure. I could be completely wrong but have a look at the clutch fork anyways

What motivated the Clutch Slave Cylinder replacement in the first place?

Cheers
Ian

 

[Mitchell B]

I would think if the seals are good, then even hitting the stop-ring shouldn't cause a leak. If that was the case we'd have leaking brakes all the time. Are the cylinders "new", or remanufactured? I know in the past when I've had to use something other than a factory MC, leaking was almost expected. Just for clarity, the leak is on the line end of the MC (front seal), and not on the pedal end of it (back seal)?

My curiosity would have gotten the better of me at this point, and I would have removed the stop-ring, pushed the piston out, and taken a gander at the seals (installed backwards, rolled, torn, etc), as well as the bore quality (pitted, scored, etc). Would look to ensure the seal is fully in front of the inlet port so that there isn't some kind of bypassing of the seal when you initially depress the pedal (bleeding difficulty/leaking).

I dont want to to mess with it, they will use that as an excuse to not replace it under warranty. It's leaking out of the bore, past the piston and into the rubber boot. I believe they were both brand new but I dont really know 100%

 

[Mitchell B]

Hi Mitchell

Getrag Transaxle? Is it possible the clutch fork inside the bellhousing has become misaligned?

If the clutch slave cylinder pushrod doesn't engage the clutch fork there is nothing to hold the clutch piston in its proper resting position to start the action of pushing against the fork to lift the clutch pressure plate when you push on the clutch pedal.

Probably won't bleed properly either and the seals may be resting with no pressure on the lip of the seal...

It's a weird one for sure. I could be completely wrong but have a look at the clutch fork anyways

What motivated the Clutch Slave Cylinder replacement in the first place?

Cheers
Ian

It's the Audi 100 5 speed.

There is a spring pushing the piston towards the clutch fork and it can't get past the internal retaining ring. I'm pretty sure it is still working the clutch but have to try and test that to make sure. Hopefully get that done tonight or tomorrow.

It was leaking before and couldn't disengage clutch so had to replace it.

 

[Mitchell B]

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

I will have to get back to you on the diameters, however it's the same as what's been in the car since 2005 so wouldn't think the issue would just show up now?