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.
Under normal circumstances, the high pressure pump supplies the pressure regulator and the main accumulator with fluid. The output from these two feeds the whole system with high pressure, going through the security valve which keeps the brake circuit constantly under pressure, for obvious reasons of security. If there is enough pressure in the system, the security valve feeds the rest of the suspension via the anti-sink valves and the height correctors.
This pressure coming from the security valve appears in the control circuit of the anti-sink valves. When the car runs, the valves are constantly open, connecting the height correctors to the rest of the suspension and brake subsystems—everything functions exactly as in cars not equipped with this anti-sink system. Even when the engine is turned off, the valves remain open as long as the feed from the accumulator remains at a higher pressure than that of the suspension. But as soon as the leakage in the struts, height regulators and the brake valve reduces the pressure in the main accumulator below the suspension pressure, the closing anti-sink valves isolate the suspension struts from the rest of the system. It is usually the front valve that closes first as the front of an unladen car is much heavier due to the engine and gearbox. Compared to a non-anti-sink car, the leakage is quite drastically reduced. For instance, a standard XM with its suspension in prime condition takes about 20-30 hours to sink completely, while with the anti-sink system this would take as much as ten days.
The rear anti-sink valve is connected slightly differently: in addition to feeding the rear suspension and the brake circuit, as usual, it connects to an additional anti-sink sphere as well. The function of this sphere is to maintain pressure in the braking circuit. As the brake valve is the most leaky element, it could exhaust the pressure between the piston and the plunger while the remaining pressure behind the piston (provided the high pressure and the front suspension circuits do not leak that much) stays rather high. In this case the anti-sink valve might open again in error—this additional sphere ensures that this will not happen.
This system maintains the car height by counteracting the internal leakage of the various suspension element that would make the pressure escape back to the reservoir. Elements that are in constant motion—height correctors, for instance—leak past their seals on purpose to lubricate themselves. The anti-sink valves—which move very rarely, need no intensive lubrication, thus are manufactured with very close tolerances and hardly leak themselves—isolate all the struts from the rest of the system to prevent any possible leakage to reduce the pressure in the struts, allowing the car to sink.