Cannot get the brake caliper bolt off my 2001 gmc sierra.?

[cramsnomme]

I am trying to replace my brakes and rotors on my truck. I have tires off and everything. I have tried using a 18mm socket wrench but will not budge the brake caliper bolt. I know I had a star bolt before now its just a 18mm 5 star bolt. I just cannot get it off any suggestions?