Water that has been boiled or has been standing about for a while has no gaseous substances in it. The compressibility of any fluid is bugger all compared to how far the brake flexes, and how much the fluid expands when its heated. Wayne, if you think that because water contains air it will be compressible, consider that any oil based fluid also contains hydrogen as a minimum. You might be an authority on certain things, stop trying to do everyone down on any point you dont agree with.