you have something in the caliper , since your going to change fluid anyway have someone pump the break and keep pressure on it and keep releasing the fluid. it should squirt out fast, when low take the bleeder out and hit one more time, put in the new bleeder and lines and bleed till full. If your not changing the pads I'd also take them out and push the cyclinders all the way back in to break any rust or dirt in the disk system.