The problem here is that all hydraulic Citroens use the rear suspenion spheres as rear brake accumulators, if you study the plumbing you will find that the pressure goes from the rear height correcto to the rear spheres via a junction, one pipe from this junction take fluid to the rear brake feed in the brake valve. This means that when you operate the brakes, the rear brakes will have a varied pressure depending on what the rear suspension pressure is, IE if you put some paving slabs in the boot, you will get more pressure in the rear suspension to help lift the car, so therefore more pressure is sent to the rear brakes for stopping power.
This can sometimes be a problem as when doing a long slow stop, then sitting at the lights with your foot on the brakes, it will sap the rear suspension of all the pressure reserve, thus making the car take a dive at the back as soon as the brake is released, then the height corrector tells the car its too low and the car comes back up. (good news the heightn corrector is working fine).
You can demonstrate this yourself. Start the engine, put your foot on the brakes then select high on thr lever, you will find that the front will operate normal but the rear will be very reluctant to go up, release the brakes and the rear of the car will jump up fast. Put your foot back on the brakes and select low, the front will go down as normal but the back will be held up by the rear brakes, release the pedal and the rear will drop like a stone. This demonstrates the effect of the rear brakes on the rear suspension.
As for the problem of dropping, this is normal but not as much as to notice it too much, If the spheres have low pressure especialy the main accumulator this can make the problem worse. The Xantia will typically wear through a set of spheres every 3-4 years.
You could try pleiades at sawtry, I get hydraulic bits there
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Hope this helps in some way.
Regards Slim