Terrible Xantia brakes

pete_wood_uk · Post by **pete_wood_uk** » 15 Jun 2005, 13:46

Thanks Simon. You were slightly off-beam [:)], but in investigating your suggestions I think we've cracked this at last.
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Mandrake
One thought here - is the accumulator sphere in good condition ?
...
As far as I know (and I don't claim to be an expert on this, so I could be wrong) the spring is quite soft, and is only there to ensure the valve will close fully when the pedal is released.
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The HDI's accumulator seems fine. It's a bit reluctant to reliably do the levelling test, but ditto with the engine running; however the regulator ticks every 90s, and you can operate the brake pedal gazillions of times with the engine off before you run out of pressure.
So I did that on both cars - dropped the suspension and operated the brakes until they ran out of pressure. I wanted to check the stength of the in-valve spring on both cars. Turns out to be (a) light and (b) identical, as you predicted. However, getting down and pushing on the pedal with my fingers while there was still pressure, the feel of the two were very different. The TD brakes were as you'd expect, whereas the HDI ones had a definite two-stage action - you push, it squidges, then it kicks back. Reproducible - squidge-kick, squidge-kick.
Anyway, having run out of pressure and found that the springs were identical, light pressure right to the end of the travel, I started 'em up again. Lo and behold, the HDI brakes were more or less fixed. Slight squidge/kick if you really look , but nothing like before. Excellent. How the hell?
I can only conclude that there must have been something like an air bubble trapped in the pipe to the reaction piston. That would give the squidge, then as the valve opened the increasing hydraulic pressure would compress it and give the kick. I dunno, just a theory. And maybe my depressurising the system and operating the valve to full excursion has pushed most of it out, or at least around into another part of the system where it's less noticeable.
This is just a thought-experiment, I have no data except a magically fixed brake system [:)].
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Indeed, it could be very ordinary things like pads etc... often people use front pads that are too hard - they last long but they don't give a good bite action...
Perhaps the calipers could do with lubricating too...in any case, now that they both don't have springs, unless the accumulator sphere is a bit stuffed on the HDi, any differences in the brakes are likely to be discs/pads/calipers etc, as the brake valves should be identical.
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Indeed. Well it's having lots of money spent on it today - two new expensive tyres - so the disks & pads will have to wait a month or two while the credit card recovers from recent excesses, I don't believe that the warp is bad enough to make it unsafe, just mildly annoying....
Thanks for your help.
Pete

Mandrake · Post by **Mandrake** » 15 Jun 2005, 14:49

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by pete_wood_uk

Thanks Simon. You were slightly off-beam [:)], but in investigating your suggestions I think we've cracked this at last.
<hr height="1" noshade id="quote"></blockquote id="quote">
Excellent [:)]
<blockquote id="quote">quote:<hr height="1" noshade id="quote">
So I did that on both cars - dropped the suspension and operated the brakes until they ran out of pressure. I wanted to check the stength of the in-valve spring on both cars. Turns out to be (a) light and (b) identical, as you predicted. However, getting down and pushing on the pedal with my fingers while there was still pressure, the feel of the two were very different. The TD brakes were as you'd expect, whereas the HDI ones had a definite two-stage action - you push, it squidges, then it kicks back. Reproducible - squidge-kick, squidge-kick.
Anyway, having run out of pressure and found that the springs were identical, light pressure right to the end of the travel, I started 'em up again. Lo and behold, the HDI brakes were more or less fixed. Slight squidge/kick if you really look , but nothing like before. Excellent. How the hell?
I can only conclude that there must have been something like an air bubble trapped in the pipe to the reaction piston. That would give the squidge, then as the valve opened the increasing hydraulic pressure would compress it and give the kick. I dunno, just a theory. And maybe my depressurising the system and operating the valve to full excursion has pushed most of it out, or at least around into another part of the system where it's less noticeable.
This is just a thought-experiment, I have no data except a magically fixed brake system [:)].
<hr height="1" noshade id="quote"></blockquote id="quote">
Your theory is quite plausable. From your description, I'm sure it is air trapped in the system somewhere, and it may indeed be a big bubble of air trapped in the pocket at the end of the shaft that forms the feedback piston.
There isn't a seperate pipe as such, it's just a bore inside the control block from the output of the valve that feeds the front brakes around to the end of the valve control shaft that forms a piston. It's been a while since I studied the diagram of the brake doseur so my memory might be a bit foggy, but thats the essence of it.
If there was a pocket of air trapped on the end of the piston then when you initially push the pedal the pressure would be immediately transmitted to the brakes, but you wouldn't get any feedback feel on the pedal until that air pocket was compressed to almost nothing, at which point the rush of oil to compress the air would cause it to suddenly "slam" the piston backwards, and that could explain you feeling a slight delayed kick back on the pedal.
Also the pocket of air would make the pedal too easy to push initially (due to the delayed feedback) and until the air pocket was compressed you'd essentially only be pressing against that very soft return spring, so your initial braking action would be excessive then fall off a moment later. (Causing loss in precision of brake application)
An intriguing idea...the question is, would normal bleeding of the brakes help here ? I suspect not, as the air pocket is sitting in a small dead end, although its not clear how your method would expel it either, although I guess if the distance from the piston to the junction between the valve and the brake line output was quite short, pressing the pedal fully may be able to expel most of that trapped pocket of air out into the brake line between the valve and the front brakes.
In any case, if it has been expelled the only place it could go would be into the front brake lines, so it would be worth the trouble of bleeding the front brakes to make sure you get rid of it entirely...
Regards,
Simon

rbruce1314 · Post by **rbruce1314** » 05 Jul 2005, 20:49

I admit it- I was wrong (see previous posts)!! Last week I was sorting out the front mats and my hand just happened to wander up to the brake pedal.... 15 mins later I had a pedal without a spring and what a difference! As well as all the other posts stating the difference to the feel, the biggest surprise was the lack of drag as you come off the brakes. Previously I would come off the brakes a few feet before where I wanted to stop and it would still stop in the desired place: now I only release at the point I want to stop at. Don't ask me why: it ain't logical but I love it [:D][:D][:D][:D]

Mandrake · Post by **Mandrake** » 06 Jul 2005, 05:34

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by rbruce1314

I admit it- I was wrong (see previous posts)!! Last week I was sorting out the front mats and my hand just happened to wander up to the brake pedal.... 15 mins later I had a pedal without a spring and what a difference! As well as all the other posts stating the difference to the feel, the biggest surprise was the lack of drag as you come off the brakes. Previously I would come off the brakes a few feet before where I wanted to stop and it would still stop in the desired place: now I only release at the point I want to stop at. Don't ask me why: it ain't logical but I love it [:D][:D][:D][:D]
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Welcome to the fold of believers [:D]
Regards,
Simon

dnsey · Post by **dnsey** » 11 Jul 2005, 18:24

How is the spring canister removed?
I want to take mine off to get access to the clutch pedal pivot bolt (guess why?), and I've spent half an hour pulling, pushing and levering it every way I can think of, but it won't budge even with a hammer and chisel (fairly gently applied!). Is it just very tight, or is there a knack to it?

Chrispy · Post by **Chrispy** » 11 Jul 2005, 18:42

I had to lever mine out with a screwdriver as it was wedged in. Get a flat blade screwdriver inbetween the edge of the canister and the hole in the brake pedal and twist. That'll get it moving.

Mandrake · Post by **Mandrake** » 12 Jul 2005, 05:05

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dnsey

How is the spring canister removed?
I want to take mine off to get access to the clutch pedal pivot bolt (guess why?), and I've spent half an hour pulling, pushing and levering it every way I can think of, but it won't budge even with a hammer and chisel (fairly gently applied!). Is it just very tight, or is there a knack to it?
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I just lifted the brake pedal as high as it would go, grasped the cannister in the other hand and wiggled like crazy until it started to move, then it pulled out. It's just a press tight fit into the back side of the brake pedal arm. A screwdriver might help too but I didn't find it necessary.
Regards,
Simon

_kid_ · Post by **_kid_** » 08 Apr 2006, 16:22

Finally got round to doing this, why I waited so long I don’t know. Easy to pull out the canister from the back of the brake pedal and change the spring for a piece of metal pipe. I thought the spring itself was fairly stiff when applying pressure by hand but applying pressure with my foot i could see the flex in the spring.

Now it feels like the pressure which previously was only available when the brake pedal was pressed a lot, now comes immediately and stops the car quicker and feel safer.

I would recommend this option to anyone who has not yet changed and of course its fully reversible if you preferred the previous state, I doubt you will though

HST · Post by **HST** » 08 Apr 2006, 22:28

Have to say this mod doesn't work for me. Gentle driving round town & on major roads the brakes were fine without the spring. On narrow, twisty roads driven even moderately quickly it was impossible to modulate the brakes properly. The slightest bump felt through the body would affect the pedal pressure and hence braking which mad me look hopeless to my passengers. It was also very unnerving for me driving, especially on wet roads.

I had the (fairly stiff) spring out for a couple of weeks before putting it back. I bitterly regretted not keeping the spring handy in the car so that I could have replaced it sooner.

My car as standard (2.1TD Exclusive) has a decent pedal feel, though not as good as my all time favourite 405 SRi. That car's pedal feel so was so good it didn't need ABS: I could easily hold the brakes close to locking on wet roads. I've never had brakes so controllable since.

The worst car I've driven for brake feel was a recent Mondeo. I scared myself at the first junction because of the massive travel before the brakes did anything. This is compared to a sprung Xantia (and every other vehicle I've driven). The worst for actual stopping was a Marina. It is worrying what can pass an MOT: roughly 50% of the stopping power of a modern car in good nick.....

BonceChops · Post by **BonceChops** » 08 Apr 2006, 23:33

I had to perform an emergency stop from 80mph to zero on the motorway today. The stopping power of the car is amazing, without spring, not that it matters when your foot is hard down relying on Mr ABS to do his business. This is with a 2.1 estate so it has the bigger discs and tyres. I am glad there was no one behind as I doubt they would have stopped so quick

awg2 · Post by **awg2** » 10 Apr 2006, 09:25

HST ,
I had the same problem as you with braking sensation in my first Citroen - a GS. It took about a month to acclimatise to the (very) different feel. Now (4 Citroens later) I feel uncomfortable driving any other car!

Mad Ax · Post by **Mad Ax** » 11 Apr 2006, 09:03

Did this job myself last night - only took a few minutes to whip out the spring and find a piece of pipe to fit. I haven't drive the car far yet - I took it on a quick cruise around the block - but it definately feels much more controllable.

I've got a 5-hour working night around central Bath tomorrow, so I guess I'll know soon enough if it's worked