push the piston back in is all you should need to do , sometimes when the rubber line bends it will put enough pressure on the fluid to make the caliper piston push out some
Push it down with your hands... LOL!!!!!
Sorry... Are they front or rear? I asume front. Your gonna want to take the pads off and use a c-clamp to push it down. NOTE: Don't do that if it is the rear brakes, you'll ruin the calipers.
You need a C-clamp and a small piece of wood/metal/etc. that has enough area to cover both pistons. With the piece of whatever covering both pistons use the c-clamp to compress the pistons, then putting on your caliper will be a piece of cake!
.... I removed the caliper in the first place because I don't want to bleed the brake.. one of the nipple already rounded out and I'm not paying $37 for a new one..
In that case, tighten the C-clamp very, very slowly, so as not to agitate particles suspended in the brake fluid and force them back through the ABS unit.