The rear caliper compresses into the bore by rotating the face clockwise. You can do this with a special tool or with long, needle nose pliers. Unlike the front calipers, the piston extends and retracts on a spiral screw attached to the bottom of the caliper bore.
You need a caliper tool and a ratchet. Caliper tool is a cube with prongs on it, a couple dollars at a parts store or online. Just turn the caliper tool and on the caliper and it will compress.
some model's that have the parking brake Incorporated into the rear caliper, you must compress and turn the piston until the proper clearance is obtained.
I have used a large "C" clamp, to compress the caliper flush to the face.
My 1999 infiniti G20 rear calipers have to be twisted clockwise in order to compress them. The auto parts stores have a tool set you can rent that will compress and twist at the same time. I went the cheap route and bought a universal cubed looking tool that attaches to a ratchet and it worked but was really hard to compress and twist at the same time. Definetly recommend the tool set rental.
A special tool (not expensive available at Sears or auto parts) is required Piston is threaded
Replace the caliper and bleed the brake system.
Assuming this is the rear caliper you are having trouble with, the piston must be turned clockwise as you gently compress it. You basically have to screw it back in while applying pressure.
You will need a caliper rewinding tool
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It requires a special tool to turn and compress the piston in at the same time.
c clamp or channel locks or depress the pedal
with a large pear of channel locks,or a caliper compressor