Hydraulic clutch help!?

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 Ram MkiV 04 Jul 2022

Lost the clutch on my OctaviaII tdi the other day, pedal went to floor with clutch staying engaged. Drove car home ok with rev matching for changes and fortunately didn't have to stop completely anywhere.  Located the source of the problem after removing significant portions of turbo pipe, air intake pipes, etc.  The master cylinder had split - could see fluid spurting out the side when clutch pressed.  Got master cylinder off with some difficulty and swearing and got a replacement.
So far so good....
Fitted new cylinder and attempted bleeding procedure but could tell something wasn't quite right.  Clutch did work and was disengaging but biting early and didn't feel trustworthy/right.  Presumed air in system.  Noted fluid reservoir wasn't decreasing - fluid was not being admitted to new cyclinder.  Took cylinder off and compared with old one.  On the old one you can blow down the feed nozzle that connects to the fluid reservoir and, when the cylinder is in the 'fully clutch out' position air passes through and exits the cylinder outlet (leading to slave cylinder)  All as you'd expect.  On the new cylinder however it does not admit air (or fluid!) through from the feed nozzle and out the exit in any position.  Hence why the bleeding failed and had air in system when first fitted.  Even at pressures of over 30psi (I got my foot pump involved to try and blast some air though) absolutely nothing gets through.

So did I miss something you're supposed to do with a new cylinder?  The vids on youtube aren't great but they seemed to get success and didn't mention anything weird you have to do to get the cylinder to admit fluid.  Faulty part?  I can't think so but I'm at a loss as to any other explanation.  I'm picking up another new cylinder tomorrow morning....I'll be surprised (and delighted) if I can blow air through it and think will have to admit defeat it if I can't.  Unless anyone can shed some light?
 

 artif 04 Jul 2022
In reply to Ram MkiV:

Piston stuck in the pedal down position. Usually a spring to push it back but when it's dry it can stick in before being bled properly.

Stick an air line on the outlet port to try and push it back

OP Ram MkiV 04 Jul 2022
In reply to artif:

Thanks for reply but don't think it's that.  I removed the new cylinder and was able to move the piston back and forth between its two extremes.  It should admit fluid/air when fully out (ie. clutch out position) but it doesn't.  Unlike the old one which does.  Unless you mean there's some sort little internal spring valve thin inside the cylinder unit? Surely not...

 artif 04 Jul 2022
In reply to Ram MkiV:

I've not seen it in detail, but Google suggests its a twin seal cylinder, the inner seal could still  be stuck 

 montyjohn 04 Jul 2022
In reply to Ram MkiV:

If the res inlet isn't letting any air in then as you suspect it's never going to work correctly.

From what you've said, it sounds like your bleeding top to bottom. If this is the case, how did you particularly bleed it? Sounds like it worked for a few pumps then somehow blocked the inlet.

I generally bleed bottom to top, so force fluid into the slave cylinder (assuming you have access to it) and fill the Res this way. Air wants to rise so it's much easier to bleed this way. Now if the inlet on the mc is blocked this won't work either but forcing the fluid this way might clear whatever is causing the issue.

I assume when you removed the piston you could blow air through the inlet to confirm it's not a casting defect.

Is the main seal deformed and blocking the inlet?

Honestly a little stumped on this one.

OP Ram MkiV 04 Jul 2022
In reply to montyjohn:

> From what you've said, it sounds like your bleeding top to bottom. If this is the case, how did you particularly bleed it? Sounds like it worked for a few pumps then somehow blocked the inlet.

To attempt the bleeding, I opened the slave bleed screw and pumped clutch pedal a few times.  Fluid was coming through but now I'd guess it was just residual fluid in the line between master and slave getting pumped out. Can't be sure but I felt it didn't draw anything from the reservoir from the off. Then did a few repetitions of depress clutch, close bleed screw, release clutch, open bleed screw.  I realised something was up doing this as even with vacuum created, master cylinder wasn't drawing from reservoir (level staying constant) and could hear/sense air being taken in at slave bleed nut when opened.

> I assume when you removed the piston you could blow air through the inlet to confirm it's not a casting defect.

No, when cylinder is fully removed and piston drawn back, cannot blow air through (even at 30psi+!)  I'm picking up another new one tomorrow...  I'll be delighted if, when I get my hands on it, I can blow through it and all the trouble's been caused by a casting defect.  (I'd be surprised though as I'd have though VW/Audi parts would have better QC than to allow such a defect to pass?)

> Is the main seal deformed and blocking the inlet?

Possibly I suppose...though it felt like the piston was sliding inside the cylinder happily enough which I guess would be indicative of seal being ok... 

> Honestly a little stumped on this one.

I'm completely stumped.  I'll be surprised if it's down to a defective part as mentioned above but equally, I'll be surprised (and very confused) if it's down to anything else.  Ready to move on in life now!

Thanks for your reply.

 birdie num num 05 Jul 2022
In reply to Ram MkiV:

You need to drive an automatic 

2
 dunc56 05 Jul 2022
In reply to Ram MkiV:

Use a pressure kit . Did exactly the same 2 weeks ago on my golf. Manual bleeding no chance. Pressure from the tyre on the kit and bingo.

 dunc56 05 Jul 2022
In reply to dunc56:

Oh and the reservoir has to be over full. That was they key or all you do is suck air in.

 jkarran 05 Jul 2022
In reply to Ram MkiV:

I'd guess one of three things:

Blocked reservoir outlet, maybe something in the bottom of it, blocked screen, mould flash or maybe it's bottoming out against a surface when fully installed. If it's a remote reservoir check the outlet pipe isn't kinked or pinched, unlikely if it's resisting 30psi of air.

Piston stuck down, pushrod not pulling it back up.

Pedal-piston linkage out of adjustment, the piston is still partially depressed when the pedal isn't. Check the pedal has unimpeded motion. If the linkage is not adjustable it could be out of tolerance parts or perhaps you've missed a spacer somewhere when installing it. If it is adjustable, adjust it.

jk

Post edited at 10:11
 montyjohn 07 Jul 2022
In reply to Ram MkiV:

What was the conclusion in the end? Any joy?

OP Ram MkiV 08 Jul 2022
In reply to montyjohn:

The parts place exchanged the master cylinder.  I could blow through this second one which confirmed the first one was dodgy.  Quite surprised but relieved by this.  Fitted new one, was euphoric to see it convincingly draw fluid from reservoir but then absolutely crushed when fluid was now pissing out of bleed screw on the slave cylinder when clutch was depressed!  Shagged o-ring seals there new problem, caused (inevitably really) by repeatedly tightening and slackening the screw on ~15yr old rubber o-rings I guess.  Managed to find an o-ring to fit main seal but running short on time (and patience) couldn't get hold of a second o-ring which is crucial for pressure bleed so couldn't/didn't do that....Manual bleed worked well enough to have a working clutch... Just got back from 1000mile drive back from France to UK in it, very relieved t it made it.  Absolute saga!  What would've been relatively straight forward fix compounded into a multi-day, confusion and stress filled epic due to that first faulty part.  Could probably now do with fitting proper o-ring seals on bleed screw and then doing a full pressure bleed of the system to be 100% but going to have a bit of break from arsing around with the clutch for a few days!
Thanks a lot for the useful comments and suggestions, helped me think it through and confirm I wasn't going insane. 

 montyjohn 08 Jul 2022
In reply to Ram MkiV:

Glad it's mystery solved.

If a job takes less than three time longer than I expect then it's normally gone well. Hopefully you're under ten time longer.

Post edited at 14:03
 artif 08 Jul 2022
In reply to Ram MkiV:

Good to here you found the problem

This guy was stuck for four days because of a faulty part and poor fitting, not so bad living in a log cabin though

youtube.com/watch?v=Q9oi2iaCsug&


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