My friend, who happens to be a Porsche racing mechanic (and very German) during the spring/summer, taught me a trick that I thought would be asking for trouble. He cracks the bleeder before he compresses the piston into the caliper which allows for the "contaminated fluid" to escape and not make it into the system. I thought this would introduce air into the lines but he made a good point...you only will get air into the lines if the master cylinder empties while you have the bleeder open. Gravity will keep the fluid draining out of the open bleeder and no air will enter. After you replace the pads, close the bleeder and top the master cylider. I have been using this technique for years with no untoward effects. You also clear out a bit of old fluid at the same time.