The sinking when standing with the engine off is due to the valve blocks of the VA and HA. There are metal valves in there. Over time, dirt and soot residue build up on it and the valves do not hold tight. If you rinse the system properly, use new oil and a new filter, it usually works again. If the oil is burnt, I advise you to switch with flushing anyway, because sooner or later the tandem pump will die. Incidentally, it is often the case that more than one strut sags. After all, the dirt collects everywhere. Everything speaks for us like dirt in the facility.