Caliper Bolt

I have a stuck RR Caliper bolt. this is the upper bolt that holds the piston housing over the rotor. I wanted to lube the bolts but I could only get the upper one to come out the length of the treads. I tried a punch to help drive it the rest of the way out but its just will not budge. The bolt turns and I can screw it back in but I cannot take it all the way out. Any suggestion on what might be holding the upper bolt into the housing.