Remove the bleed valve on the caliper take a rag and put against the caliper across from where the piston will exit. Use compressed air and a rubber tipped air nozzle- press the nozzle to the bleed hold and hit it with 120psi and it will pop right out. The hard part is getting the pisotn back into the cyclinder wtih the dust cover around the piston (again lower pressure compressed air is the key) .
Trust me on this, trying to replace the seals on calipers today is a losing proposition. With new and rebuilt capipers available in most price ranges the question is why would want to expend the extra time to do what the rebuilders have already done for you? They do it on a daily basis with the best of equipment and expertise, which in all cases will be far more precise than anything the average DIYer could accomplish. Brakes are the means of stopping your vehicle and not the place to cut corners to save a few dollars.
To answer your question however, most use compressed air to force out the pistons once the caliper is off the car. Bear in mind too that in many cases when the piston seals need to be replaced the piston bore will need resurfacing also. Do you have the means to do that?
Either pump the pedal when the caliper is still attached but not on the rotor,
or when off an air line can be used to blow the piston out.
In extreme cases heat the caliper to "burn" the seals so the piston is loose.
Good luck finding the seals, They quit making wheel cylinder kits available years ago due to incorrect installation leading to complete brake failure. Caliper seals and pistons same way. 'Course, this in in the States.
The braking system on your vehicle uses brake calipers to clamp down the brake pads against the brake rotor, slowing down the vehicle in the process. These calipers have at least one piston that extends or retracts with the pushing and releasing of the brake pedal. These pistons have seals around them, and those seals can break down. Should that happen, the caliper will need to be replaced or rebuilt. If you decide to rebuild the caliper, you have to remove the piston from the caliper, which takes about 30 minutes to do.
