Any work on the hydraulic parts below the car should be carried out with the car safely supported on axle stands or standing over an inspection pit. Never work or allow anybody to work on the car (not even for a small adjustment or even just inspection) when it's only the high setting of the hydraulic system that keeps the car up. Even a slight adjustment of the height control linkages or any other failure can cause the car body to drop many inches and easily crush anybody causing death or very serious injury. This is not a theoretical warning, it has already happened. Even with the car safely supported, always watch out for the possible drop in height and allow sufficient room for it to happen.