isetta

Fiesta 1.4tdci mk6 2004 clutch pressure problem

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My sister's car. Owned by her 2 yrs, problem started about a year ago, about 140k miles. No paperwork with car, no idea if anything replaced in past:

Fault: Clutch pedal loses pressure and goes slack.
(This is not the common problem where the master cylinder pushrod slides off the pedal. It has in the past had a hole drilled and a split pin inserted to stop that happening).
It seems to only do this when the car has been driven on a longer journey and therefore hot.
When it does this, the pressure can be got back by pumping the pedal, this is more successful if the car has been left a while to cool down. It can then be OK for a few journeys.
 
To try to fix I replaced the clutch master cylinder - this was before we reckoned there was a link with heat - (bought from Euro car parts) and bled it, some months ago. It seemed OK for a while but now has the same original problem (it might be that it was only used on short journeys for a while after replacing the cylinder. I have bled it a couple more times since but the problem still happens now and then.
 
Each time I have had a look the fluid level has not gone down and I can see no sign of a fluid leak anywhere. 
 
Of course I can not see the slave cylinder. But if the slave cylinder is at fault my theory is that it would be leaking and hence fluid level would go down (and probably see fluid leak at bottom of bellhousing).
Whilst I have done work on cars in the past many many years ago including changing clutches I have never done such work on a car before with one of these modern slave cylinders inside the bellhousing so i don't claim to be an expert on them.
 
I'm all out of ideas. She took it to a garage who just want to replace the clutch and showed little interest in checking the car and considering if that will fix it. Approx £400. She doesn't mind paying this if it is going to fix the problem - it is a very nice car other than this problem - £30 road tax, excellent mpg and not had any other problems,  but I am not confident it will fix it because I can't see any logical explanation of how it is the clutch itself, although i admit I can't think of a logical explanation of any other part causing it either. Particularly as I think the engine being hot plays a part in it, and it not always doing it. 
 
I am worried she is going to pay £400 (+ vat probably) and find the problem persists and she really can't afford to chuck that money down the drain.
 
I am hoping someone else has had this problem and can tell me what the fix was, please help if you can. I did find another post with same kind of problem but no resolution.

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I am having exactly the same problem with my Grand C-Max 2016. Did you get anywhere with this?

My mechanic just changed the master cylinder, and it is still happening. I have exactly the same symptoms, only happens longer journeys. If it's being driven on the motorway for 10+miles it's particularly bad. The pedal pretty much drops to the floor and needs pumping to get any kind of workable pressure back.

 

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On 6/11/2020 at 7:33 PM, isetta said:

It seems to only do this when the car has been driven on a longer journey

 

27 minutes ago, kjubb said:

If it's being driven on the motorway for 10+miles it's particularly bad. The pedal pretty much drops to the floor and needs pumping to get any kind of workable pressure back

If master cylinder and reservoir problems have been ruled out, that leaves few options. There is only one pipe from master to slave (at least so I believe), so if fluid is not leaking from the slave end, it must be being pushed back into the reservoir.

If there is some odd fault in the clutch actuating system, that gradually pushes back the slave piston much further than normal, then when you press the pedal, it will go straight down, quite possibly several times until the slave piston has been pushed back to its normal operating position. This is most likely to happen over a longish distance without any clutch actuations, maybe with heat to help reduce friction in the slave cylinder, So a mechanical, dirt or wear related fault inside the clutch could well be the cause.

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