Could be the brake hose, many times they collapse internally, you can usually tell by simply opening the bleeder when the brakes are stuck, if it then releases it is in the lines, if it does not release it has to be in the caliper or pad mounting area. Did you clean the rust from under the stainless steel clips???? did the pads go in free??? If it were further up the hydraulic system both rear brakes would be sticking. Also remember, just because a part is new doesn't make it good!!!!!!!!!