Hydractive centre sphere, large effect on brakes ?

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Mandrake
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Hydractive centre sphere, large effect on brakes ?

Post by Mandrake »

Hi All,
Those following some of my posts will know that I've been chasing down "dead" feeling brakes that lack "bite" on my 1997 VSX Xantia and although all the normal things (pads, bleeding etc) have made worthwhile incremental improvements, they still havn't been quite right - until now. [:)]
I already suspected a punctured rear hydractive centre sphere so today when a new accumulator sphere arrived, I thought I'd try it as a temporary hydractive centre (the pressure is a bit high, 62 instead of 50, but otherwise compatible) and sure enough the old sphere was punctured, and much to my surprise not only did the soft mode ride suddenly spring to life, but the BRAKES improved dramatically! [8)]
The deadness and lack of sensitivity is gone, and more importantly the response time seems much faster, with more "bite". Now I can easily trigger the ABS into action whereas before I could barely brake hard enough to get any response from the ABS.
It seems that on a Hydractive 2 system, if BOTH the anti-sink sphere and the rear centre hydractive sphere are faulty, then the back brakes don't work properly, and respond very slowly.
Initially I was a little bit puzzled but after looking over the hydractive 2 diagrams again I think I can see what happens.
On an old Citroen with no anti-sink valve/sphere and no hydractive, the rear brakes run entirely from the rear corner spheres, no problem there, as the damper valves are not too restrictive.
Enter hydractive 2 and anti-sink - in a Hydractive 2 the damper valves on the corner spheres are VERY restricted with slow flow rate, especially in the extension direction, which is the direction the oil must flow out of the spheres to provide braking action.
Furthermore, the oil from the outer spheres must flow not only through the very restricted damper valves in the spheres, it must also flow through the additional damper valves in the hydractive block to reach the brakes.
On the other hand, in the soft mode the oil from the centre sphere is able to flow to the rear brakes without going through ANY damper valves, (therefore fast action) and effectively acts as a second brake accumulator to help the anti-sink sphere when in soft mode.
In hard mode, the centre sphere will be completely cut off from the brake circuit due to the hydractive control plunger, leaving only the corner spheres (with their stiff damping valve) through the restricted ball valve, and the anti-sink sphere.
This is a problem, because as soon as you hit the brakes hard, the computer switches into hard mode, thus disconnecting the centre sphere, and if your anti-sink sphere is flat, you get poor brakes in hard mode.
So in summary, some possible failure modes:
a) Centre rear hydractive and anti-sink both flat - very poor rear brakes, with slow/weak/unresponsive action when the suspension is in either soft or hard mode.
b) Anti-sink flat but hydractive centre ok - normal brakes in soft mode, slow/weak/unresponsive in hard mode.
c) Anti-sink sphere ok - normal responsive rear brakes regardless of the condition of the centre hydractive sphere or hard/soft mode selection.
So it seems that the anti-sink sphere on a hydractive 2 model *IS* important while driving, not only as an emergency brake source if the anti-sink valve is closed, but as a source of reliable brake pressure if the centre hydractive sphere is faulty, and when in hard mode.
Comments anyone ? Anyone else noticed a big improvement in brake response after replacing a rear hydractive centre sphere ? I don't know the actual condition of my anti-sink sphere at the moment, but I suspect its fairly low in gas...making my rear centre hydractive sphere the main source of supply for the rear brakes.
The moral of the story is that if you want your brakes working properly on a hydractive 2 model, you MUST check the anti-sink sphere and rear hydractive centre sphere...much more so than a non-hydractive 2 model, where the anti-sink sphere serves no purpose during normal driving operation.
A lot of people only ever change an anti-sink sphere if the car starts going down too fast at the back, and don't realise it might be fairly important for the rear brakes even while driving, not just in an emergency if the main pressure supply dissapeared...
Regards,
Simon
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Post by jeremy »

Well done Simon - always nice to find the problem by elimination, and in reality you are probably one of a handful of people in the world who really understand the systems - pity you can't turn your knowledge to profit!
Sreiously its a pity there isn't an easy way of testing spheres. I know one can build a hydraulic test rig but its quite a lot of work, probaly needing some expensive bits like a pressure gauge and its also large for a device that will get occasional use. There are also the safety implications of working with extremely high pressures which will add to the cost as proper joints and components rather than bodged ones will have to be used.
I can't think how to work out what the gas in a fully charged sphere would work out to be. Even if one needed to weigh the sphere before fitting due to inconsistencies in construction weight would it be great enough to make weighing a method of determining charge or am I dreaming?
jeremy
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Post by DoubleChevron »

Sphere testers are a peice of cake to build, however you still need to remove the spheres to test them.
http://www.aussiefrogs.com/shane/sphere ... tester.htm
seeya,
Shane L.
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Post by Mandrake »

<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>

Well done Simon - always nice to find the problem by elimination, and in reality you are probably one of a handful of people in the world who really understand the systems - pity you can't turn your knowledge to profit!
Sreiously its a pity there isn't an easy way of testing spheres. I know one can build a hydraulic test rig but its quite a lot of work, probaly needing some expensive bits like a pressure gauge and its also large for a device that will get occasional use. There are also the safety implications of working with extremely high pressures which will add to the cost as proper joints and components rather than bodged ones will have to be used.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I know what you mean. Me and Dad are putting together a tester again, but its not as hard as it seems - everything you need can be made from spare Citroen parts - a spare pressure regulator, hydraulic pump, a few pipes and fittings etc. The only extra things needed are an oil tank (we're planning on using a 1 litre coffee tin [:D]) and a pressure guage. (Already have one)
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">
I can't think how to work out what the gas in a fully charged sphere would work out to be. Even if one needed to weigh the sphere before fitting due to inconsistencies in construction weight would it be great enough to make weighing a method of determining charge or am I dreaming?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Yes you are [:D] Unfortunately the empty weight of the sphere is unknown, unlike an empty LPG bottle, and the weight of the gas is so small compared to the weight of the sphere that it would be impossible to measure it accurately.
An oil pressure test is the only way of doing it...other than various other guestimate approaches such as the sit-in-the-boot test for the accumulator, and a bounce test for suspension spheres etc...(complicated by hydractive 2 hard/soft modes etc)
Unfortunately a bounce test is only a reasonable method if you're familiar with that model of Citroen and have some idea of how soft it should be in the first place - which is usually not the case for a first time Citroen owner, who is exactly the kind of person that WONT have a sphere tester...and someone who has the experience to build the sphere tester and use it probably knows the car well enough to tell by a bounce test if the spheres are a bit flat or not.
And of course the other problem is actually getting the damn sphere OFF the car. [:(!] It took me well over an hour today trying to get the rear centre hydractive sphere off (complete with spare wheel carrier removed) even with the help of a second pair of hands. Time to make a better sphere removal tool I think....
Regards,
Simon
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Post by bernie »

Well done Simon
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Post by rg »

Yep, good work, Simon! Thanks.
I now need to find an easy way of accessign the anti-sink on my S2 2.5 XM :-(
rg
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Post by citronut »

well it is recomended by GSF when replacing front spheres on an active or activa you should fit new centre sphere at the same time,because if you dont you may still have stif susp,by the by i think you will find rear susp presure returns through brake dozer valve not other way round,as rear brakes are fead directly from dozer valve
regards malcolm
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Post by Mandrake »

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

well it is recomended by GSF when replacing front spheres on an active or activa you should fit new centre sphere at the same time,because if you dont you may still have stif susp,by the by i think you will find rear susp presure returns through brake dozer valve not other way round,as rear brakes are fead directly from dozer valve
regards malcolm
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Hi Malcolm,
Naturally its a good idea to replace the 3 at a time, but in my case when I got the car the suspension is in an unknown state - and it turns out the centre rear sphere is completely punctured but the corner spheres are ok, so obviously someone waited until it went rigid, replaced the corner spheres, and called it a job done without doing the middle one....
As for the brakes, I never said that the rear suspension pressure went *directly* to the brakes, obviously it goes through the brake pedal, or you wouldn't have any control over them [:D] When I'm describing the back suspension and saying "to the brakes" I'm implying the connection that goes up to the doseur valve and then back to the brakes...(sorry if that wasn't clear)
Regards,
Simon
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