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
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
Hope this helps in some way.