front caliper issues 92 maxima

I am replacing the front brake pads on my 92 Maxima, and I cannot compress the caliper piston back in at all. Right now I'm just using a big clamp pliers with an old brake pad against it, but it wont even budge with all my strength, which is really weird. The lid to the fluid reservoir is open, but i really don't know what else I can do never had this problem. Any suggestions? Could this be indicative of a further issue with the caliper?