1999 Chevy Suburban caliper not moving

I have replaced the front rotors, calipers, pads, etc. while bleeding the brakes i can not get the passenger side caliper to move at all. I am getting fluid through the lines and it bleeds normally, but after I close the bleeder valve and try the brakes the caliper is not moving at all. I pulled everything apart and was able to move the piston on the caliper(thought it was seized) any ideas what is going on? I am beginning to belive it is in the abs system, maybe a bad check valve? Any help would be greatly appreciated. Thanks in advance.