ya, u need to constantly fill the resavoir when you are doing this. What you are looking for when you are using the vavuum pump is clear (new) fluid coming through the lines. Once you are sucking clear fluid through each caliper, the system is bled and all new fluid is in your lines. You MUST make sure the the resavoir is full the whole time or you will suck air into the lines, and that could take a while to get out. When me and my friend bled his brakes, we definately didnt take a liter out of any of the calipers. Just enough fluid that the lines can hold basically, and that isnt that much.