Fluid is used because it doesn't (in a practical sense) compress - elevation change wouldn't mean a thing. As I mentioned previously, transportation usually means changes in orientation and jiggling, which can often lead to air getting on to the business side of the cylinder. Laying my bike on its side usually doesn't induce a problem if I'm driving an hour or two, but I usually need to flick the handles after longer drives, like our annual family road trip to VA.