Many contemporary Citroëns—including both regular hydropneumatic and Hydractive Xantiae and XMs—have an anti-sink system (SC/MAC) fitted, to keep the car from lowering when not used. The system does not interfere with the normal functioning while in use. It attempts to minimize leaks inside the system by having only one element that can leak, the anti-sink valve itself.
The introduction of this anti-sink valve coincided with the appearance 6+2 piston high pressure pump. As the suspension is fed from the smaller, two-piston side of the pump, pumping the car up from the low position would require a lot of time (although its performance is perfectly sufficient for the normal operation once the car is already running).
To avoid this scenario, the anti-sink valves fitted for each axle between the height corrector system and the suspension struts (or the hydraulic control block on Hydractive systems) keeps the car body from lowering when the engine is switched off. The valves operate on the pressure differences in the system, without any electrical control: when there is significant pressure in their control circuit, they keep their work circuit constantly open.