Doing a brake job and the caliper piston will not push back in to make room for new pads? There are a few specific reasons, and the fix depends on which one. Here is why a caliper piston will not compress.
Rear pistons must be twisted
Many rear calipers with an integrated parking brake have a piston that screws in - it will not push straight back. You must turn it clockwise while pressing with the right tool; see caliper compression tool. Forcing it straight in damages the mechanism.
Open the bleeder or reservoir
As the piston goes in it pushes fluid back up the line; if the reservoir is full it resists, and a clogged line makes it worse. Crack the bleeder screw while compressing (with a hose to a bottle) - this eases it and avoids pushing dirty fluid back; see reservoir overflow.
A seized piston
If it is a front piston that should push straight in but will not budge, the piston is likely seized from rust or a torn boot; see why calipers stick. A seized caliper needs replacing - a loaded unit is easiest.
Do it right
Identify whether it is a twist-in rear piston, then use the correct tool and crack the bleeder. If it is truly seized, replace the caliper; see new pad won't fit. Fit quality pads and hardware.