Slave Cylinders aren't user servicable. The seals can go on them, but it's quite unlikely. You'd also notice a loss of fluid. Take the pads off and see if there is oil on the back of them.
Try and find out where the noise is coming from - for something to make a noise like you describe, you should be able to see play in something. What do you mean by there is play in the pistons?
Maguras can make a thud when the pad contacts with the rim, particularly if the wheel isn't moving at the time.
Jon