Xantia sinks like a stone

[superloopy]

You mean once the car is stopped? Or with it running? Is there anything to show what each pipe relates to?

[xantia_v6]

Could possibly be the doseur, it's worth checking that return line as well.

You can check all of the returns easily at the LHM tank

It won't be the doseur, as the anti-sink valve isolates that (unless both are faulty). With the engine running and the car at normal height there should be no significant flow in the return line(s) from the back of the car or the brake valve.