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Sometimes what happens is these things gets seized up or have debris in the way. I would recommend cracking the bolt to the brake fluid. If it still will not go in you will need to replace the brake caliper. Sometimes when they are seized up you can force them free but I do not fee this is safe practice when dealing with your brakes. Also you should be pressing the piston into the brake caliper until it is flat and all the way in.
You can use a C clamp that has a little more power to it. That is normally what we do. But cracking the brake line should make it easy enough that you can almost push it in by hand. After you will need to bleed the brakes but sometimes this is necessary on replacements if the caliper is being stubborn.
Guidelines for bleeding When reconditioning or replacing brake callipers bleed the relevant brake pipe as follows. The brake pedal must be depressed throughout the operation. This is so that the brake system is not drained of brake fluid. If the braking system has been completely or partly drained, bleed the whole system. Note! Use only genuine Volvo brake fluid with designation Volvo DOT 4+. Never reuse brake fluid. Always keep brake fluid in original packaging and well sealed. Preparations Switch off the ignition. Clean around the brake fluid reservoir filler cap. Take out the container. Remove the filler cap. Fill the brake fluid reservoir completely. Raise the car Bleed left front wheel Note! While bleeding, check that there is sufficient brake fluid in the reservoir. Top up if necessary. Remove the protective cap from the bleed nipple. Connect the hose from the collection bottle. Depress the brake pedal a few times. Maintain pressure. Open the bleed nipple. Depress the pedal fully. Keep it fully depressed for approximately 5 seconds. Release it again. Repeat this 3-5 times until there are no air bubbles in the fluid coming out of the hose. Tighten the bleed nipple. Remove the hose from the bleed nipple. Install the protective cap. Continue bleeding the other wheels in the same way in the following order: Right front wheel, left rear wheel and right rear wheel
I would first attempt like I stated above opening the brake line on the caliper if the piston retracts without to much trouble than you have no problem at all.