Get a rebuilt one. Let the pros do this.
While you are bleeding, push the brake caliper cylinders back into the caliper by squeezing them with large pliers, such as channel locks. This cleans and lubes the cylinders. You don't have to take the pads out.
If one of the calipers is bad, this will often times demonstrate the problem. Just as a caustion, obviously, never pump the brakes with the pads out of the caliper or a disk between them or you will hyper-extend the cylinder. (You don't want this ....)
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)