I have now replaced all four suspension units, all "O" rings, overhauled the compressor and the solenoid block, replaced the desiccant in the air dryer, and checked (many times) for any apparent leaks (none found now). The vehicle suspension works fine when driving it, and whilst the engine is on - it rises to the highest setting, drops to the low setting and rides happily at the "normal" setting. However, when I switch off the ignition, and park up, the suspension will drop to the stops. Sometimes it does this in a very controlled and quick manner, with the solenoids operating on and off. At other times, it will take three or four days. I have now put two valves on the front air supply, and shut them prior to switching off the engine. The suspension stayed up, with absolutely no drop at all, over a period of days. Does this mean I still have some sort of problem with the solenoid block?