high frequency vibration on the brake pedal

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alexx
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Post by alexx »

Hm ...
When I bought my Xantia, rear brake circuit was full of N-gas, that obviously had escaped from rear spheres, because they were on 1/4 of the initial pressure. I soon recognised the problem, because rear end was rising probably up to rebound stops, just like if you brake with the handbrake, until after several seconds it dived, when rear brakes started to grab. But there were no strange effects on the pedal, only higher pressure was needed to stop.
Anyway, it's possible that oscillations occur somewhere in the doser, as I said a while ago.
I'm not quite sure that Anders described function of doser valve correctly. When the system is under pressure, first two sliders will always return back when you release the pedal, even without aid of springs. Both pistons have a bore leading to their bottom, so the pressure from under is equal to the pressure in brake circuit, and this pressure will return them back. They can't be misplaced.
Third slider is pressure limiter for rear brake circuit. When certain braking force (determined by the load on the rear axis) is reached, second slider will come in contact with it. Until that point, pressure in both circuits is equal (braking force is not, because rear brake pistons are smaller). After that point, pressure in rear circuit will rise more slowly then the front.
I didn't notice any variation/reduction in rear braking force during braking - the car stays in level from the very begining, so front and rear brakes must be grabbing the same moment (now, after bleeding rear circuit). Only on heavy braking, rear end is rising a little - obviously because of rear brake limiter in action. And, since rear end is not rising nor dropping, pressure in rear suspension also stays the same, despite of the load transfer to the front, and the pressure limit for rear circuit is roughly constant during braking.
Or so I think ...
PS. Don't believe to every word in Nastasic's/Gabor's Citrone guide.
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AndersDK
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Post by AndersDK »

"Or so I think ..."
May just as well be my words Alex [:)]
About the springs between pistons : they are only there to initially keep the pistons correctly located - they don't actually play a role in the valve's function as you correctly points out. But would certainly crumble and then do other nasty things if the mechanical endstop was not fitted [;)].
I'm still struggling to 100% understand the rear brakes circuit piston. It's clear what you say that the piston would move on higher boot load - since the rear suspension pressure feeded would then raise. The hysteresis I keep rubbish about on this piston is simply the difference in pressure that makes this piston first move - and then release again.
It's all down to understanding which actual pressures are present at both sides of piston under various conditions.
Tx your comments - can't live without comments - as then I'm stuck in wrong anticipations on the funtions [:o)]
alexx
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Post by alexx »

Some explanations in the Citroen guide are really too complicated, so I'm asking myself is author, although very competent, completely understanding the subject.
This 'hysteresis effect' is unimportant here in my opinion. It happens (if it happens at all) when the system gets pressurised, not when you press the pedal. At that moment, third slider is moved to the bottom in my opinion (pressure in rear suspension is at least 60 bar; according to the picture, it acts on the area about 1 cm2, and from my remembering, 600 N force is enough to compress the big spring and move 3rd slider to the bottom). When you press the pedal, first two sliders are moving towards the bottom, third slider moves in the opposite direction until it come into contact with second slider. From that point, pressure in rear circuit will be rising more gradually than in the front circuit. On older, 2 slider type doser valve, this pressure is 28 bar below the pressure in rear suspension. From that point on, pressure in rear circiut is rising at about 1/4 rate of that in front circuit (for the same 2 slider type doser), until pressure in rear suspension is reached. It's probably the same here, just slightly improved construction.
Some more data (I measured this 6 years ago; take them with some tolerance):
diameter of slide valves: 7.5 mm
clearance between 1st and 2nd slider: 1.5 mm
clearance between 2nd and 3rd slider: 1.5 mm
clearance between 3rd slider and bottom: about 3 mm
Springs between sliders are really weak (like in a pencil), they are probably there to keep 2nd slider at place when you are not pressing the pedal, to prevent from constant draining of the rear suspension. Only spring between the cup beside 3rd slider and bottom of doser is stronger and mounted with some preload.
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Post by bxbodger »

I have an old copy of "car mechanics" magazine which contains a Xantia buying guide, full of pictures of dirty LHM and collapsed suspension, which claims that high frequency pulsing through the brake pedal just happens in some Xantias and is perfectly normal, and shouldnt put anyone off buying one!!!
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Post by Homer »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by AndersDK</i>

1) Air in the hydraulics : Citaerobics. Note that "air" could be N-gas from a slowly leaking rear suspension sphere. This would be indicated if Citaerobics cures the problem first off - then re-appears a couple of days later. the sphere can leak for months before it actually goes flat.
2) Weak spring device between pedal and brake valve. This device was incorporated on Xantia models to make the hydraulic Citroen brakes feel like a "normal" car.
3) Flat accumulator sphere.
4) Defective brake valve.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I had a problem which sounds similar to the original post. Vibration, similar to having a wheel slightly out of balance but only when braking gently from high speed. In my case I knew what the culprit most likeley was becasue I had recently swapped the front wheel s to the back and the back to the front. Swapping them back cured the problem.
Which suggests one of the rears has a problem but as they are alloy and the spare is pressed steel and there is no vibration with it on the rear I'll live with it.
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AndersDK
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Post by AndersDK »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by bxbodger</i>

I have an old copy of "car mechanics" magazine which contains a Xantia buying guide, full of pictures of dirty LHM and collapsed suspension, which claims that high frequency pulsing through the brake pedal just happens in some Xantias and is perfectly normal, and shouldnt put anyone off buying one!!!
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
- also seen here locally in an advert : "Nice BX in good overall shape, no rust, new MOT, all brakes renewed, new pipes. Suspension a bit hard, but this is normal on these older Citroen cars, runs great ...... "
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