I had this problem one time and somehow while working on the truck I had dented one the brake line that ran to the right side. If you can't find anything wrong I would change out the other wheel cylinder. Maybe the reproductions are made a little diffent and the preasure just wont match up. it seams to me that if there was enough of a difference in the bore size inside the cylinder then they would never move the same amount at the same time. I could be wrong and Im sure if I am someone will point it out
