Brake fluid is hydroponic - absorbs water...hence why you never want to use old fluid you find lying around the garage. Anywho, after months or years of use, the fluid absorbs water through the brake lines -minute amounts albeit...either way, the moral is brake fluid has a very high compression point, whereas water does not. You put water (or air) into a brake system and the pedal will be spongey. To correctly flush and fill your brake system, do the following...get a syringe to suck out all of the old fluid from your resivor,wipe it all out with a CLEAN rag...any particles left will jack up your system. Fill the resivoir with brand new clean fluid and then proceed to bleed the calipers, starting with the right rear, the left front, the left rear and finally the right front, their is a reason for this, but id have to type a lot more than i want to. once you see clean fluid, and no air bubbles while you are bleeding said caliper, you are done, continue with the next caliper. (DONT FORGET TO REFILL THE RESIVOIR HERE AND THERE) if you run dry at any time you have to start the process all over. Once complete, you will have a very firm brake pedal....