It is threaded-They make a special tool (NOT EXPENSIVE) available at Sears
The Brake Piston Cube is a cube-shaped tool used to retract brake caliper pistons in vehicles when changing brake pads. Its design allows it to fit into the caliper housing and apply even pressure to retract the pistons back into the caliper, facilitating easier brake pad replacement. By turning the Brake Piston Cube, the pistons are compressed uniformly without damaging the caliper or seals.
The piston retracts by turning it back in. You will need a Disc Brake Pad Spreader (Part Number: 648412) and a Disc Brake Piston Tool (Part Number: 648410) found at Advance Auto Parts. The Brake Piston Tool looks like a Rubik cube. Simply line up the knobs into the piston slots and place the disc brake pad spreader between the caliper and brake piston tool. Apply slight pressure on the piston via the pad spreader and turn the brake piston tool with a screw driver while continuing to apply pressure on the brake piston tool. From what I have read they could retracted by turning either clockwise or counterclockwise.
If the piston is just a solid or hollow cylinder, you can use a pair of big channel locks and an old brake pad to squeeze it back in, if the piston has little cut outs in it you will need to rent a tool that twists the piston back in to place, like a bolt.
I don't know what car is but all the cars I've worked on I simply used a flat screwdriver wedged between the old brake pad and the calipers piston, or pad and rotor to pry the piston back far enough to fit the new pad, it's never failed to work.
Possible that the rear sensors are broken.
Brake pad changing is important in maintaining a car. A person lifts the car, removes the tire, brake shoe and old brake pad. Next put the new brake pad , put the brake shoe and tire back in their place, and lower the car.
The back brakes on my Honda Accord are disc brakes. After removing the brake cylinder assembly you will see the brake cylinder piston end has a slot in it. The short pin on the back side of the inner brake pad fits into this slot, but also it is used to 'Turn' (clock-wise) the piston back into the cylinder so you fit the assembly back over the new 'thicker' pads. You can not 'press' this piston back into the cylinder.
Easiest way is to use an old brake pad and a C-clamp. Put the pad over the piston and crank the clamp. If you don't have an old pad, use a small piece of wood to cover the piston.
pad brakes are disc brakes, it's known as changing brake pads for disc setup and changing brake shoes with drum brake setup.
you take out the 2 bolts that mount the caliper to the caliper mount. Then you take a small pry bar and push the brake pad closest to the caliper piston so you can get the caliper off, if it doesn't just slide off. then you take one of the old brake pad and a vice grip clamp, you put the brake pad in front of the piston and put the clamp on the brake pad and the back of the caliper and clamp it down until it will fit over the rotor with the new pads on.
assuming that nothing else is wrong. when changing the brake pads use a big C-clamp or some vice grips to retract the cylinder. if there are two cylinders you may need to put an old pad between the clamp and the cylinders to push them both back in simultaneously.
Changing brakes is pretty simple. A person has to lift the car with a car jack, remove the tire, brake shoe, and old brake pad. Then put in the new brake pad, replace the brake shoe, put the tire back on the car, and lower the car.