changing brake pad on the back

You can either use a special brake piston compressor tool or a just an old brake pad and a c-clamp to slowly compress the piston back in.

I used a small Pry Bar and pushed back the Pads only a little. Once the pads are off you will need a special tool. The Calibers on the back have the emergency brake built into them and the piston part of the caliber will need to be turned counter clock wise to allow the space for new pads. You will see the part that holds the back of the pad and it has two slots on it. The piston can't be pushed back far it auto adjust clockwise. Good Luck

Retract Rear Brake Piston on 2005 Mercury Montego (Ford Five Hundred) The rear brake piston on the 2005 Mercury Montego has to be rotated while being retracted. On the driver side, the piston must be turned clockwise and on the passenger side the piston has to be rotated counter-clockwise. This is important. It is difficult to apply enough compressive force to retract the piston by using just the cube tool which is turned by a 3/8 inch socket drive. If you do not have a tool that both rotates and compresses the piston at the same time, you can use a combination to the cube tool and a simple screw compressing tool (or C clamp). First use the screw clamp to exert compressing pressure on the piston. Then rotate the piston in the correct direction using channel lock pliers. Keep rotating the piston while you apply more compressing pressure with the screw clamp. When the piston is sufficiently retracted so the cube will fit, you can use the cube. Before doing this, find the side of the cube that fits the indentations on the piston head and grind off the knobs on the opposite side of the cube. Place the cube against the piston head and use the screw clamp to push the cube against the piston. As more pressure is applied by the screw clamp, turn the cube in the correct direction using an adjustable wrench on the outside of the cube.