OK, the 'slack' you feel is the space before the leaver actually starts pushing the fluid through. So what you need to do is adjust your TPA (red dial) unil that slack is taken up. Then, if the brake is fine you're good to go, if it's still pulling too far down, you need to ajust the tpa even more, however if you adjust it too much, you get the problems with the lazy pistons you are talking about. If that is the case, unwind the tpa so the pads fully retrct back to the pistons (this is the same point when you have just taken up the 'slack' you talked about). Then, you need to undo the clamps that hold the pistons and move them closer to the rim, you need to have the pads pretty close to the rim, about a pennies width. Then, tighten up the bolts and see if that's any better. The idea of a magura is to only use the tpa to take up the slack, and then only use it a couple of twists beyond that point. If your tpa (red dial) isn't working, post back here and let us know and we can link you to a topic on how to fix that.