New Master Cylinder and can't get air out of one brake line

I had this same issue. Turns out, the new wheel cylinder was leaking. Pull back the rubber wheel cylinder cover that has the short rods that push out on the brakes. If there's fluid in there, in front of the inner rubber cup, that means the wheel cylinder is leaking. I ended up putting an old wheel cylinder back on. A lot of newer parts are junk. Also, you should have gotten a bleeder kit with your MC. Maybe the auto parts store sells them.