Xantia anti sink sphere

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Nikolaymk
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Xantia anti sink sphere

Post by Nikolaymk »

My car is now 9 yrs old and has done 59K miles. I don't think the anti sink sphere has ever been changed. Should I replace it, what's it's function and what can go wrong if it's not replaced?
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AndersDK
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Post by AndersDK »

Hi Nikolay -
The antisink sphere is directly connected to the pressure input of the brake doseur (pedal) valve's rear brakes circuit.
This pressure in turn is derived from the rear suspension, when the antisink valve is open, i.e. when engine is running and normal system pressure is present.
When engine stops, the system pressure slowly drops to a point where the antisink valve closes off the rear suspension, by way of pressure differences. Then the rear brakes pressure feed from rear suspension is also isolated, and no pressure exists to make the rear brakes work.
But the antisink sphere is still connected to the rear brakes pressure feed, and the sphere is always charged to the same pressure as existed in the suspension, before the antisink valve closed. Thus the sphere remembers if you have any boot load.
The antisink sphere is then seen as a brake accumulator sphere for the rear brakes, supplying a spare pressure when engine is not running.
The antisink sphere has no influence on the antisink feature itself.
What could happen ?
In extreme situations it's important you get that extra brake force from the rear wheels, to help stop the car while still in perfect balance from the brakes.
This you do not get with a flat antisink sphere, as the rear wheels would never help braking the car, with no system pressure (engine stopped).
In fact it's the same situation as on front brakes, if the main accu sphere is flat.
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Post by JohnD »

If you do decide to replace the anti-sink, don't just grab hold of it and unscrew. If you do, you'll snap the tubing going into the back of the sphere. Hold it in position and undo the union at the rear of the sphere (9mm if I remember rightly) Once the union is loose, screw off the sphere. On reassembly, enter the union before screwing up the sphere. The suspension will be in the lowest position, so it's necessary to have the rear of the car (safely) raised higher enough to work underneath.
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AndersDK
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Post by AndersDK »

Oh YES - YES - John [:0]
Tx completing my answer [:I]
BTW : Shane L has been there done that, and made pics on the job, available here : http://www.aussiefrogs.com/shane/xantia ... pheres.htm
Lot's of other interesting Citroen stuff on Shane's homepage !
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