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Disco tdi brakes - Saga!!!!


joe1
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One of those problems that goes on and on.... Auto 300tdi non- abs.

Brake pedal has always had too much travel so with mot coming up decided to fix. Pedal is firm w. engine off - so no air in system (power bled already) but when engine starts pedal sinks - as it should do but also sounds like there is air (ie 'squishy' and is only solid about half way down) present. Stopping power is ok ish but cant lock wheels up on standard tyres

So far: 1. partly sounded like mcylinder so swooped out for known good one ( but against that pedal would have felt wrong with engine off).  2. Second thought was vacuum problem so tested pump which held -20 Hg steady. Line to servo has no leaks. So servo had to be faulty - Nooo checked with another known good one and problem remains. 3. Bulkhead area where servo mounts is ok. Mount for mcylinder on servo had little bit flaky paint so cleaned back and used rtv for good measure. 4. clamped off calipers and problem remains. 5 Swapped out the pressure reducing valve just in case - rear brakes bleed up fine anyway though. But problem remains. 

So before I set fire to it...... any thoughts will be most welcome! TIA Joe

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There is also a slave cylinder (or whatever we're supposed to call them these days), which was the cause of very similar symptoms on mine.

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4 minutes ago, SPendrey said:

There is also a slave cylinder (or whatever we're supposed to call them these days), which was the cause of very similar symptoms on mine.

I don't know - do you mean pressure valve on the nearside wing?

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Sorry, my mistake... thinking back it was the servo in my case, although you say you've tried that already.  There is no "slave" either, that's on the clutch!  I had to do both at the same time (for different reasons), and my memory is obviously rustier than the car is. 🙂

Note to self: pay more attention before writing stuff!

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Sounds like the problem is at the calipers in one form or another, seized pistons, worn out pads (on metal), rusty or badly grooved discs etc, or perhaps the flexible lines have failed, as above.

Take each wheel off and check, the only way, while you are there, check the wheel bearings for play, as they can affect pedal travel.

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I’d get someone to pump the pedal hard while you check for ballooning of the flexible hoses.

The servo should hold vacuum for a fair while after shutting off the engine, and allow a few pedal operations before becoming ineffective.  If you need the engine running for it to work at all, then there is a leak.  “Known good ones” sometimes let you down.

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On 5/10/2021 at 10:00 PM, SPendrey said:

Sorry, my mistake... thinking back it was the servo in my case..... 🙂

 

I shall be re-checking that so thanks for the hint...

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On 5/11/2021 at 8:37 AM, Alsace_rangie said:

flexibles?

Pads moving freely, pistons not jamming?

I'll check flexis. Are you saying caliper possibility even though the problem is there with the flexis clamped off? Bowie said the same though.....

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On 5/11/2021 at 8:46 AM, Bowie69 said:

Sounds like the problem is at the calipers in one form or another....

Possibly but I thought I'd eliminated calipers by clamping off the flexis - and bcos problem was still there I concluded problem had to be higher up in the system than calipers?

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Think you have two problems to be honest, even with excess pedal travel, the brakes don't work very well, as I understand it?

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1 hour ago, Bowie69 said:

Think you have two problems to be honest, even with excess pedal travel, the brakes don't work very well, as I understand it?

That's true - with engine on there is excess travel with a squish noise that sounds like air but isn't (bcos pedal is high and firm with engine off).  Doing the servo test on start up with foot on brake means pedal drops about an inch or more - so servo appears to work (as it did on the previous motor it was on). And then on the road it stops reasonably but not with the neck snapping force that you want (well ok not neck snapping but as good as you get with a disco). Last mot the tester did a brake road test and said brakes were ok but needed bleeding - which isn't the case as I say. And now seem slightly worse if anything. In fact I've swapped front calipers out as well now but will look at the rears shortly....

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On 5/11/2021 at 12:57 PM, Snagger said:

I’d get someone to pump the pedal hard while you check for ballooning of the flexible hoses.

The servo should hold vacuum for a fair while after shutting off the engine, and allow a few pedal operations before becoming ineffective.  If you need the engine running for it to work at all, then there is a leak.  “Known good ones” sometimes let you down.

That's interesting about checking vac after switching engine off. I'll recheck but I think what happens is that after switch off it takes a couple or three pedal presses to get back to the hard higher pedal - which is what you'd expect?

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Something like that, maybe more, but it should be progressively harder with each push, not a sudden change.  But if you run the engine for thirty seconds or so, you should be able to switch it off, leave the car for hours and then come back and have an easy pedal for a few pushes before it firms up.  If it is hard straight away, then the vacuum is already gone and that means the servo is leaking.

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  • 1 month later...

Well..... none of the above was the cause actually! The brake mcyl can attach to one of 2 holes in the brake pedal. It shouldn't make a difference but when I moved the attachment point to the lower hole the brakes became good again - the lower hole does create more of a straight connection for the mcyl pushrod but other than that I'm at a bit of loss to explain what was going on..... I don't even want to think how many hours this little item cost - it would be far too upsetting, but at least the brake system got a good looking at...  

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