How are you going to overhaul a brake caliper? Still mounted on the car, first remove the brake pads. Then press the brake so that the pistons press out and press against the brake disc. Then disassemble the brake caliper. In the vice you 'share' the brake caliper and remove both pistons.
Then remove the sealing rings with a screwdriver. Also the tin mounting ring with which the dust cover is attached. If one - or both - pistons are stuck, drill an 8 millimeter hole in the center of the piston, tap the thread and screw in a (long) bolt. Tighten this bolt until the piston comes out. Pre-spraying with WD40 penetrating oil works wonders. Keep turning until the piston is completely out of the housing. It's a horse remedy, but it works! Then clean the brake calliper very well and especially internally. Never 'work' the fitting surfaces of the two halves with sandpaper or steel brush! With compressed air. 'Blasting' is not such a good idea, because abrasive always remains in the interior, and the mating surfaces are also damaged. Then mount the sealing ring and the dust cover with seal. You cannot 'decorate' used brake pistons with sandpaper, no matter how fine. So you take new ones and preferably made of stainless steel or stainless steel. No rust to be seen, then clean it thoroughly. Then lubricate the piston with (blue ATE) brake grease. And certainly not with the grease with which you lubricate, for example! You press the piston - straight - into the brake calliper with the utmost care. Forcing is not an option! There are two sealing rings between the two halves of the brake calliper. You must always replace it. Then screw the two halves back together and tighten the bolts with a torque wrench to the torque specified by the manufacturer. Fit the brake hose to the brake calliper and fit it back on the car - with safety lock if applicable. Then it is a matter of filling the reservoir of the brake system with the appropriate fluid and bleeding everything, starting with the brake furthest from the main brake pump. So with a left-hand drive car rear right, with a right-hand drive car so rear left. Don't forget to keep and maintain the fluid level during - and afterwards! As far as the brake fluid is concerned, it is wise to decide in advance which specification you want to add. That strongly depends on the use! Disc brakes that are vitrified, too thin, warped, badly rusted, grooved, or blue should be replaced. Consult your workshop manual! If, despite the workshop manual and our explanations, you are not sure of your case, even if you have just a little doubt about your capabilities, leave such an overhaul to a professional! In addition, you can 'your' Auto Motor Klassiek never hold his or her editorial member liable in case something goes wrong. We are not responsible for your key!