DISC BRAKE dragging

1990 4.0 auto.

Passenger side front brake is dragging pretty bad, pulling me to the right as im driving down the road.

I have replaced rotor, pads, caliper, brake hose and proportioning valve. The only things that are left are the brake lines and the master cylinder. Could a bad master cylinder cause only one side to drag? It doesn't seem to make sense to me. Any help would be appreciated!

I'm interested too cause my buddy is still having problems with his and he has done all the same stuff.

He is putting on a new brake booster and master cylinder this weekend.