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The piston requires a tool that will allow you to rotate the center as it screws in and out. You do not need to remove the park brake spring. Remove the two caliper bolts and rotate the caliper up and allow it to hang.
NOTE: Ensure that one of the 2 slots in the rear brake piston face is positioned so it will engage the nib on the back of the inner brake pad.
Make sure the new pads look just like the old pads. The pads should fit into the caliper mounting bracket securely and then you put the caliper over them. Make sure the springs on the pads get compressed when you put the caliper back on. A helpful hint is to put a lug nut on it to hold the rotor in place. The brake Spring you should not have removed, will require some vise grips and strength. Use safety protection when doing this.
That is correct. see the photos. They are of a 2003 mustang rear brakes that I did in my shop.