Ongoing clutch hydraulic problems

[J.R.]

I posted a couple of months ago that my clutch master cylinder failed on the way home from the engine remap when I was giving it the beans to try the extra power, I limped home & after bleeding had a good pedal feel but it failed soon after on another full throttle run.

 

I replaced it with a VAG part from TPS, the seal had disintegrated on the old one due to its proximity to the DPF and having and using the extra power had finished it off.

 

I then was getting similar problems with the new cylinder under test runs, they were heat related and using VCDS doing just one full throttle acceleration run the DPF temp went above 700°c, I added some rockwool behind the aluminium composite space blanket insulation and stopped the test runs, its been OK since I returned to France but other than the very rare overtake & the regens no real heat has been generated.

 

In the last few hot days it has deteriorated again and it now happens without even driving the car, I get in the car the next morning or even a few hours after parking and the pedal goes half way to the flooe with no resistance & the bite point is very low, despite the huge pedal return spring it will only return half way, I have to hook my foot under it to get it back up, once done the pedal pumps up and it drives OK, last night I pumped it back up & did not start the engine, this morning its the same deal, I think the piston has gone tight in the bore preventing the pedal returning but cant work out why it has the free travel, there is zero fluid loss, the cylinder is a strange type without the internal spring & poppet valve, it has one single pressure seal and also a seal on the pushrod, it fills by gravity from the brake fluid reservoir.

 

I have replaced the clutch bleed block having opened out the retrun flow restrictor valve & thought that might have created the problem, the concentric sleve cylinder is 100% leak free and I have pressure tested it overnight, I thought it may be a high revs crank end float problem pushing fluid back up to the reservoir but have now ruled that out as the problem occurs overnight or within hours without the engine running.

 

I did buy another new cylinder as a spare as I thought I might have problems here where I dont have an equipped workshop.

 

There is only one other possibility and that is there are 2 or 3 odd annular shaped things in the hydrauilic lines which may possibly be  accumulators, they are shown on this parts diagram but do not have a seperate part number so there is no description of them, you will see them in each of the clutch lines, they function as a connector elbow but there is absolutely no reason for them to be the massive size that they are, does anyone know what they are and what their function is? Its a pig of a job replacing the master cylinder & even though I am now experienced the job has to be done blind & by feel this is not the place for me to lose the seal, I dont want to do it if the problem might be one of these unidentified things in the clutch fluid line.

 

https://skoda.7zap.com/en/cz/yeti/yet/2015-779/7/721-721030/

 

Many thanks

[J.R.]

By the way the diagram shows the external non concentric slave cylinder, mine is the internal concentric one & I think there is one less of those donut things in the hydraulic line.

[J.R.]

Anybody?

 

Number 2 master cylinder will be fitted tomorrow unless anyone can put forward any ideas, its a pig of a job and I dont want to do it for nothing.

 

The pedal pumps up and then behaves as normal when driving, leave it for a couple of hours & it drops to the floor with no resistance and does not return despite the really heavy pedal return spring, lift it up, repeat once or twice, pump the pedal and all is well again.

 

Even if you dont drive the car and leave it in constant daytime temperature it will be the same in a couple of hours.

 

I will try giving it another bleed through.

 

The system could not be any simpler, there are no valves or springs in the master cylinder, the piston seal is static in the bore like a brake caliper, the piston is a close fit in the bore and displaces fluid when pushed forwards, when fully retracted it goes back beyond the port allowing fresh fluid to drain down from the brake master cylinder, I have tried loosening the filler in case the vent was blocked.

 

I can't get my head around this problem and those donut type couplings definitely have some purpose and could be the key to this enigma if I knew hat function they performed.

 

Anybody?

[Schtum]

I'm afraid I can't offer any solutions but the symptoms are similar to the ones that we experienced on my son's 2007 Octavia 4x4 Estate.   The gearbox had a terminal failure and the replacement took ages to source and fit.  During some of this time, the clutch hydraulics were left open.  When the new box was fitted, the clutch pedal behaved in a similar way to yours.  From memory, it was worse in one season; either summer or winter but it's a number of years ago now and I can't remember the exact details.  

 

Edited 5 April, 2021 by Schtum

[J.R.]

That is very helpfull indeed, I am hoping that your memory will be jogged and that you will tell me that it happened in summer.

 

Was your son able to live with it and continue to use the car over several years?

 

I stripped the new master cylinder and may have found a design fault that would cause the problem at higher temperatures, I am going to glue the pushrod retaining collet into the piston as under certain conditions cause by the bleed block restrictor valve it could come out meaning the piston does not return fully which would case the problems I have had.

 

Other cluse are that the new cylinder fitted in the depths of winter did it a couple of times but only when I was doing back to back full acceleration runs on my favorite quiet road test track.

 

The problems only started a week ago with the first of the warmer spring days, the time I turned my central heating off.

 

And today while we are back to freezing weather its fine.

 

Its a long shot but your experience gives me more optimism so thankyou for taking the time to respond.

[Daveyeti]

I'm having the exact same problem. Clutch master cylinder has benn replaced, same problem persists. My local Skoda dealer now suspects slave cylinder located inside the bell housing. This means gearbox off job. I just cant afford this right now. Its a big job. I'll just have to run the car and hope until I can afford the repair. Will keep you posted.

[J.R.]

I can guarantee that it wont be the slave cylinder if there has been no fluid loss, (Editted in 2022, I was 100% wrong with that comment!!!)

 

I can also tell you how to test it and prove beyind doubt that the slave cylinder is not faulty.

 

(Editted, again in hindsight that is 100% incorrect as you will see at the end of the thread)

 

Remove the air filter housing to access the clutch bleed block, have someone depress the clutch pedal and put a hose clamp or mole grip onto the flexible pipe leading to the bleed block, this will block the clutch fully disengaged.

 

Leave it overnight and then try to push the car in gear, if the car moves the clutch is still disengaged and there will have been no fluid loss, you can then confidently rule it out and avoid throwing even more money at the garage who is at as much of a loss as you.

 

I have just stripped the 2nd new cylinder again and looked very carefully at the pushrod retainer plastic collet, it is only retained in the piston by a tiny ridge of plastic, no more than a few thou and that is without any wear on the ball joint that articulates, add a bit of heat, some wear, the clutch pedal spring returning the pushrod  quicker than the restriction in the bleed block will allow the piston to return and the collet will withdraw partially from the piston meaning it cannot return fully and the bore cannot refill with fluid.

 

If you release the clutch pedal very quickly, sidestep it if you can, if it returns fast with a "clack" when it hits the stop then the pushrod is withdrawing from the piston, it should come up slowly with a damped motion and not impact the pedal stop if all is well, unless the bleed block has been replaced with a full flow performance one.

 

At the moment its a hypothesis but everything points to it, when I fit the second new cylinder where I have glued the collet into the piston (its drying as we speak) then I will know for sure.

 

All 3 of our vehicles are 4x4 but I think that may be co-incidence unless the 4x4's are the only one to have the torque limiter in the bleed block to protect the clutch from drag starts.

Edited 9 August, 2022 by J.R.

[J.R.]

My problems started when I was doing fast gearchanges during full throttle runs testing the results after re-mapping during which time the DPF was throwing off loads of heat.

 

They returned with the new cylinder when doing the same thing, then after re-bleeding etc and driving normally it behaved and the problem returned with the warm weather, it is not present today at 5°C.

 

The old one which I cut apart not having then worked out how to release it without an Octopus should still be in my UK workshop, I reckon when I get back to the UK and can check it I will find that I can easily pull the pushrod out of the piston.

Edited 5 April, 2021 by J.R.

[J.R.]

Is it only the 4x4's that have the 6 speed gearbox with concentric clutch and more importantly the clutch torque limiter (bleed block)?

 

Or perhaps the non 4x4's have a bleed block without the torque limiter built in?

 

The so called "torque limiter" slows down the rate at which the clutch is released so that if you do a drag start it will slip rather than risk  damaging the driven plate or the drivetrain, you may have on occasions had to drive out of a junction sharpish to avoid oncoming traffic, you knew you had 4x4 so no chance of wheelspin but noticed clutch slip and thought your clutch was on its last legs, it wasn't it was the stupid torque limiter coming into play.

[J.R.]

3 hours ago, Daveyeti said:

I'm having the exact same problem. Clutch master cylinder has benn replaced, same problem persists.

 

How much does it cost for a main dealer to replace the master cylinder?

 

Having looked back through historic threads on here and other Skoda forums I see a lot of people have had the problem but never found the cause because the dealer had replaced everything either all in one go or on a parts bingo basis, master cylinder, bleed block, slave cylinder but also rushing them for a new clutch and dual mass flywheel for the latter.

 

That amounts to a hell of a lot of money

[Daveyeti]

£360 inc VAT. The info you've shared is really useful. You're correct. No fluid loss which reassures me its not the slave cylinder. The new master cylinder seems to have improved things somewhat but find I'm hooking my foot under the clutch peddle to lift it back every morning. With the information you have kindly taken the time to share there is no way it could be the slave cylinder. I'm back to work tomorrow after a 10 day break so my Yeti is back to daily use. I'll just have to see how it things go. 

[J.R.]

@Daveyeti @Schtum

 

I am 99% sure that I have found what the problem is, how to resolve it for free and also how to live with it.

 

Editted, wrong again in hindsight!!

 

The plastic master cylinder pushrod is attached to the metal piston by a segmented plastic collet and the tolerances of the mouldings are not what they should be, the collet has to be fitted to the ball end of the pushrod then both inserted into the piston where a tiny raised ring should clip home into a tiny groove, the mouldings can be such that it either does not engage properly or not well enough and can come out especially when hot.

 

This gives a situation of the clutch pedal with a very strong return spring whose pushrod is not attached to the piston and the collet can slide in and out laterally.

 

The stupid clutch torque limiter means the slave cylinder releases slowly with the fluid going through a tiny orifice, the master cylinder piston will also return slowly but if the collet has displaced or is free to move then the pedal will return as quick as you remove your foot from the clutch making a noise when it hits the stop, the piston will not come up all the way as it has parted company with the pushrod, the fluid port to the reservoir remains closed, the next time you use the pedal it sinks half way to the floor and only returns half way, the collet and pushrod having moved away from the piston.

 

If the pedal is stuck halfway down then lifting it with your foot is counterproductive, it will leave the piston behind and each time you pull the collet out it wears a bit more, I will explain what to do later.

 

To resolve the problem or to prevent it occuring and/or to live with a collet that is detached from the piston you need to remove the bleed block/clutch torque limiter (an easy job I will describe later) and drill a 4.5mm hole through the centre of the bore, there is a 4.5mm bore that will centre the drill for you, you might want to buy a second hand one from Ebay to play with.

 

Once this is done you will have a very firm clutch pedal with no free play, I had to move my seat backwards! Under certain conditions the pedal may still get a small amount of free play, max speed acceleration runs snatching gears, perhaps some end float on the clutch at high revs, possibly first thing in the morning, all you need to do is depress the clutch and bring the pedal up slowly and all will be good again.

 

Its important to do this when you park the vehicle at the end of the day, if the piston does not return fully then the fluid from the reservoir cannot enter the system, as everything cools a partial vacuum will pull the piston back & the next morning you will have the old sinking non returning pedal again, do not hook it up with your foot, instead push it to the floor & let it return slowly, you may need to do this twice.

 

I am going to leave my vehicle at this stage for now and wait for the hot weather to see how it behaves before replacing the cylinder once again with one where I have glued the collet into place, I believe then I will have resolved the problem definitively.

 

The cheap (€18 inc delivery against £135 from Skoda) master cylinders have collets that dont engage, the pushrod of the 2nd new one I bought today came away in my hands, the collet then went into the piston but wont come out without destroying it, the pushrod has to go in the collet & then both pushed into the piston with a retaining glue or loctite, I think they are VAG Q.A. rejects.

 

Thats more than enough words, I will do another posting of how to remove, modify & replace the bleed block.

 

I think the problem starts or is exacerbated when the master cylinder seal starts to break up from the heat from the DPF, these particles find their way through the bleed block and further restrict the tiny return passage, the fluid will be black at this stage as mine was, VAG even make the seal magnetic and have a very strong magnet on the end of the piston to try and trap the particles, mine was covered in detritus and there was no sign of the seal, the cylinder will still function by deplacement without the seal which is a static one like in a brake caliper.

Edited 9 August, 2022 by J.R.

[J.R.]

Removing and modifying the bleed block is easy.

 

Remove U trunking between air filter and slam panel airbox, disconnect MAF sensor cable & pull off vacuum pipe.

 

Release spring clip & pull away inlet pipe & MAF sensor from air filter.

 

Undo 5mm hex socket screw holding lower part of air filter, leave top cover in place.

 

Give it a good yank to release the rubber AV mounts & wiggle it out, the clutch bleed block is now exposed.

 

Clamp off the flexible pipe, release the 2 spring clips and remove the unit.

 

The only tools for the above work are a 5mm hex screwdriver and slip joint pliers for the air trunking spring clip.

 

Drill 4.5mm hole through bore of bleed block and blow out plastic swarf.

 

Refit and remove hose clamp.

 

Remove dust cap and turn bleed nipple 180° counter clockwise by hand (plastic knurled nut) close off once fluid comes out.

 

Refit the airbox etc and you are all done!!!

[Daveyeti]

Thanks for your detailed info JR. I think you have nailed this problem. At the momment I'm leaving things well alone as the only issue I now have is the clutch peddle not returning as it should on first start up in the morning. As you have suggested, I've stopped hooking my foot under the clutch peddle as there is little point. The peddle then returns normally after first couple of gear changes. The car is then fine for the rest of the day. I'll post again if there are any further developments. Many thanks.

[J.R.]

That is pretty much where I am, the clutch after the initial couple of operations is fine but not 100%, you and I have forgotten what that feel like!

 

Mine was 100% after swapping for the modified torque limiter & bleeding but now when "fine" has that small amount of reserve travel.

 

I am hoping that when I replace the master cylinder with one where the pushrod is 100% solidly attached to the piston that things will be 100% once again.

 

To that end I have ordered yet another cheap cylinder (I screwed up the last one ) to modify, I will refit the best of my modified ones and also do an autopsy of the VAG one that I fitted about 2 months ago now I know what I am looking for and how to get them apart, which is not at all easy until you work out the trick.

[J.R.]

Having spent an evening stripping and modifying a pair of clutch master cylinders I am now 100% certain of what the problem is and how to overcome it.

 

Editted, 100% wrong!!! - following the wrong direction.

 

It is 100% definitely due to the plastic master cylinder pushrod pulling out of the piston and then becoming a sliding fit in its location, unless the two are firmly locked together the piston will not return properly and new fluid will not be drawn into the system when required.

 

It will first occur when the cylinder is heated from the DPF, its more apparent on the higher powered engines, and will be aggravated by the piston returning slowly due to the clutch torque limiter, the heavy pedal return spring will pull the pushrod partially out of the piston, from that point on the problem will be progressively get worse.

 

If the pedal fails to return the first thing to do is clean and open up the bore of the clutch torque limiter, do not pull the pedal up you will make things worse.

 

If the problem persists you need to then replace the master cylinder but you must modify it before fitting, you will probably find that the pushrod can easily be pulled from the piston by hand, the retaining collet has to be glued in with a strong adhesive, I am using PU gel construction adhesive, a trial with Loctite was not successfull.

 

Getting the cylinder apart needs a magician and its taken me a month to finally find how to do so, there is a plastic bush that needs a dozen internal spring loaded lugs compressing, it can be done with a 2.5mm or smaller tywrap or lockwire, I doubt that a VAG dealer will have ever done so.

 

Thats a very simplistic explanation and the real problem is that it has to be assembled blind, too much glue will mean the collet does not seat properly & its scrap or will gum up the works.

 

To test if a cylinder is working and fit for service you must pull hard on the pushrod and it should not detach, with it fully withdrawn you should be able to blow through the reservoir fill port and there be no restriction.

 

I have had 2 new cylinders (aftermarket) fail when I tested them by hand and the expensive VAG cylinder failed under high temperature operation, when I remove it I expect to find that like the aftermarket cylinders the pushrod collet is not mechanically retained.

 

If VAG had simply fitted an internal return spring like any other master cylinder has this problem would never have occurred, old school Girling cylinders dont have the pushrod connected to the piston, its an open ball and socket, if a vehicle is laid up & the piston rusts & does not return fully you get the same problem.

Edited 9 August, 2022 by J.R.

[J.R.]

The reason that the pedal may go half way to the floor first thing in the morning is that when the engine cools & the system would want to draw fluid from the reservoir the piston is not far enough back to allow the flow so the partial vacuum instead pulls the piston further down the bore.

[J.R.]

I removed the suspect VAG cylinder today and replaced it with a modified pattern part one where I had bonded the pushrod retaining collet into the piston to prevent it withdrawing as I thought had happened to the new 2 months ago VAG one.

 

I should have known better than to replace all the interior trims, the knee airbag and its mounting plate etc before bleeding and testing the clutch

 

After bleeding the pedal felt lighter than usual but no free play, I actioned it a few times and it sunk to the floor with no resistance and didn't return

 

After bleeding again I did a blind fingertip investigation and found that the plastic pushrod had broken by the ball joint where it clips into the pedal, my way of fitting the cylinder without removing the pedal (the return spring makes it impossible to put back) may have strained & cracked it.

 

So I have to do the job all over again for the Nth time

 

I did an autopsy on the VAG cylinder and could find no problem with it other than some grease around the seal (probably used when they assembled it) which may have been blocking the fluid top up port, either that or it was the DOT 5.1 fluid turning to silicone solids from either heat or maybe contamination. I did however find that the pushrod collet was 100% firmly retained into the piston whereas the pattern part ones came apart with a slight pull.

 

So I will be refitting the VAG cylinder because its the latest revision and seems to be better made than the pattern ones, maybe the pushrod is stronger also.

 

I wish that I could be confident that this will be the last time I have to change the cylinder but I cannot. I am getting quite adept at it though!

[J.R.]

For all my "I am 100% certain I have resolved the problem................." optimism I have been kidding myself, the problem remains and has progressively worsened.

 

I will be changing my clutch, DMF and concentric slave cylinder next week, after 3 months of trying everything to avoid pulling the 4wd transmission unit out I no longer have a choice as I have to bleed the clutch after every short trip into town, a 100 mile round trip involved losing the clutch on the way & having to bleed it, the same on arriving at the destination and once again during the return leg.

 

Air is getting in at the lowest level either the concentric cylinder or at the bleed block O rings (which have been changed for 2.0mm dia cross section from 1.8mm) but there is zero fluid loss, not even a drip

 

I am really hoping that not only will this finally cure the problem but that I will be able to find what exactly has been happening, the only remaining theories that I have are axial oscillation from a worn DMF and/or a fractured preload spring on the cylinder.

 

The air never rises above the bleed block as it sits in the vertical bore under the bleed valve, clamping the flexible line always results in a solid pedal, but beware, only push the pedal with your fingertips or you will bend or break the plastic master cylinder pushrod and have the pleasure of yet another master cylinder change.

 

So the only thing that I can be conclusive about for now is that the air must be getting in at a lower point than the bleed valve, I hope once everything is removed I will find out where, how and why.

[J.R.]

The new clutch kit arrived today, below is a photograph of a new old stock genuine VAG part concentric slave cylinder with beside it the new and enhanced user experience plastic one from the kit, both are Luk manufactured parts, my vehicle has a plastic one fitted from new, until now I could only see the stub of the connector external to the bellhousing.

 

Now I have a good idea what lies within the bellhousing there is no prize for guessing where an air leak might come from

 

[skomaz]

Looks like a potential fault waiting to happen...   granted there'll be a seal in there but even so...

[J.R.]

I noticed that the stub which protrudes for the bleed block to connect to was very wobbly, I bodged a tywrap retainer to pull it all up tight & stop the bleed block from moving around.

 

When I secure the new cylinder to replicate it being bolted in place & try to move the stub on that there is no movement in the joint, I think the old one has been shaken around over the years and developed movement which in turn has rubbed against the O ring seal.

 

I will hopefully know next week when I remove the old one. I may have spent €320 on a clutch & DMF for nothing but I wasn't going to not replace it while the gearbox was out.

[J.R.]

Well the job is done now, a very heavy transmission to remove weighing 68kgs!!!

 

The thrust bearing was faced with plastic where it contacted the pressure plate fingers and had indentations, I dont think this damage was relevant to the problem but both my new release bearings had metal contact faces.

 

The DMF had slight wear and the lateral oscillation may have contributed to the indentations on the release bearing or vibrated the fluid joint allowing air ingress, it was silent at low revs as is the new one but at high revs its much quieter, now its a diesel sound rather than a high rev death rattle.

 

I decided to fit the aluminium cylinder so as not to risk a future problem with a 1ct O ring requiring transmission removal, I think my car may have been originally fitted with an alu cased bearing & that the clutch has already been replaced, some bolts were loose, the starter solenoid connector broken and other indications that the job had already been done by an inexperienced or slapdash mechanic.

 

Also someone had abused the clutch, pressure plate & flywheel had burn marks, the pressure plate was dished, 1.5mm over a 40mm wide contact face and the drive plate worn thin on the outside only.

 

The clutch and its hydraulics are behaving perfectly since replacement, also the gear selection is much improved as the counterbalanced quadrant that the cables attach to was loose on the selector shaft spindle (more signs of being worked on), so loose that the spline would soon have stripped and I would have lost all gears

 

 

 

 

 

 

 

Edited 21 June, 2021 by J.R.

[J.R.]

The two pieces old plastic cylinder with the damaged joint allowing air ingress were made of different plastics, the inner part black glass filled nylon, the outer a yellow plastic, the new plastic cylinder (which I didn't fit) has both parts made of the stronger glass filled nylon.

 

Given that the air ingress happened when things got hot I think differential expansion also played its part.

 

I wonder if the Sachs cylinders are alu and not plastic or plastic but in one piece without the joint?

[J.R.]

More than a year later on and 20k very tough miles towing overloaded removal trailers and everything is still fine and dandy with the clutch, no air drawn into the system.