Replace - do not rebuild - wheel cylinders. They are cheap enough you should just buy them. When you try to rebuild them, they can have a problem "catching" in the bore, which will result in the problem that you describe. The bore that has the "caught" piston will have less brake pressure - in this case the right wheel cylinder.
Also, if you have access to a brake pressure gauge - check the brake pressure at each wheel cylinder. This can tell you if there is a problem with the plumbing leading up to the wheel cylinder. It helps a lot in brake diagnosis.
