WARNING..Brake pressure loss.............?

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tudor
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WARNING..Brake pressure loss.............?

Post by tudor »

Hello people,further to my (Hissing XM)last week. I Have just noticed
this message comes up on the dash when I start the car.
Having just had the serviced,a month ago and having had 1 sphere replaced on the front and all the others were checked and recharged where necessary.Also brakes all round were done and car still hisses.
Could this brake pressure warning be related to this hissing noise.
Regards Tudor,
2.1TD XM Estate 95
jeremy
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Post by jeremy »

Is this the normal notice that comes up when you start the car before pressure has built up or another?
For all the theory look at Gabor's textbook in the links section - it will explain how it all works.
Also have a look at the GSF pricelist. I see your dealer claimed to have re-gassed spheres where necessary. How much were you charged? - look at the price of new spheres.
Jeremy
tudor
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Post by tudor »

Hi Jeremy, yes when I start the car there is a small dot matrix screen
in green. Anyway these little messages come up ,which I assume are normal.It goes something like, Hydraulic pressure- Normal susspension-
Stop test lamp. Now these come on as the car rising up to it's normal position and then when it's there I get a final message.
Warning brake pressure loss.
Now having said all that ,the car drives really well apart from the hiss.
Regards Tudor
alan s
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Post by alan s »

I know absolutely nowt about an XM but warnings like that on Citroens often have a common denominator; have you checked the level of the LHM???
Alan S
jeremy
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Post by jeremy »

The strange thing is that the brakes operate at a lower pressure than the suspension, and on a BX for example the sequence is that the safety valve operates and extinguishes the warning light before the car rises.
Jeremy
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AndersDK
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Post by AndersDK »

Jeremy -
This may point to a dodgy safety valve switch - or a simple cable problem from this switch (common on BX'es) ?
alan s
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Post by alan s »

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

The strange thing is that the brakes operate at a lower pressure than the suspension, and on a BX for example the sequence is that the safety valve operates and extinguishes the warning light before the car rises.
Jeremy
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I'm thinking with a BX about when you go round a corner & the warnings light up and with my CX, it will light up a similar warning with low fluid yet still be driveable.
Won't take too much effort to prove/disprove the theory.
Thing is that with an XM being a later more sophisticated system, it's feasible to think they could "overkill" in the warnings departmrnt by fitting say a level switch high enough to warn prior to total fluid loss & associated total failure or alternatively to warn if the filters were partially clogged which was starving the pump intake. This could also explain the hissing sound. I'm basing all this on the fact it had spheres replaced and/or regassed which often results in a lowering of the fluid reserve.
Alan S
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Post by alexx »

About brakes operating before suspension:
When you start the engine, pressure of the LHM in the accumulator sphere will immediatelly reach the pressure of nitrogen in the sphere (like in a pressure-tester). When it's new, it is 62 bar, when it's old, it may be close to zero. Then, HP pump will fill LHM into accumulator sphere, compressing nitrogen in it, and the pressure will gradually reach 175 bar. In each moment, maximum output pressure for front brakes is equal to current pressure in the accumulator sphere. So, when you start the engine, front brakes are operative, but the maximum braking force is limited (or very limited, if accu sphere is low on the pressure).
It's different only on vehicles equipped with DIRAVI, since there is additional accumulator sphere for front brakes.
jeremy
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Post by jeremy »

The point about priorities is that the safety valve which operates the low pressure warning certainly on BX's allows fluid into the front brake circuit until its operating pressure is reached when it opens and allows fluid into the rest of the system. This takes place at 80 bar perssure and its fully open at 100 bar.
The warning light switch is operated directly off the valve plunger which should increase its reliability. On a BX it is normal for the stop and brake warning lights to come on at startup and go out when sufficient pressure has been obtained. What I don't know is whether your light is the simple BX type showing in a screen or if there is some element of computer activation. Some computer type units in fact provide a circuit failure feature and if for example a wire drops off the light will come on. This ties in with what Anders is saying.
As a first step have a look at the security valve (on BX first part of circuit after regulator on left side at tear of front subframe) and see if the wires are still on it and making good connections.
jeremy
tudor
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Post by tudor »

Cor blimey, thank you guys for your suggestions I have been away a while,
nice to come back and find some good info to follow up on ,will keep you posted.
Regards Tudor.
tudor
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Post by tudor »

Just to keep you all up to date,took car to an Independent cit mechcanic and it turns out to be the spheres.I bit the bullit as it were,changed them all. Bingo! no more hissing noises.
Regards Tudor
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