One method of extracting pistons from a caliper is to force compressed air through the hole where the flexi-hose was fitted. You’ll need a suitable air tool, such as a duster gun. Wear goggles and see the next two steps for more details on specific types of brake caliper.
Single piston calipers are easier to strip than multi-piston calipers. Hold a thick cloth over the piston and make sure your fingers won’t get trapped when it pops out. Feed the compressed air into the caliper via the flexi-hose attachment and the piston should be forced out.
Forcing the pistons out of multi-piston calipers is not so easy. All of the pistons have to be extracted progressively. Use a piece of metal/wood or an old brake pad to prevent one piston popping out fully. Once all the pistons are fully extended, they can be extracted with pliers. In some cases, the caliper can be unbolted to split it in half and extract the pistons.
After extracting the piston(s), extract the outer dust cover/seal with a pick or a small screwdriver. This is usually perished and split, so it will let moisture inside the caliper – it should always be renewed.
Extract the main seal inside the caliper using a scribe or a pick. This can be awkward to do, but avoid losing your temper, which could result in scratching or damaging the inside of the caliper where this seal is fitted.
Once the caliper has been stripped, clean inside it and all of the piston(s) using a parts wash or white spirit. Avoid using anything abrasive to remove corrosion. If the piston is corroded for instance, and cannot be cleaned with a solution, replace it.