On the output of the rear pressure regulator and arms in valves, the air is tee'd to both rear arms. Provided your pressure is set to a normal range, it sounds like your right rear arm may be out of level. If you remove the air to the rear arms, if you place the right rear in a position where it is almost all the way out, does it stay in that position without air, or does it fall toward the center more than the left? Bring the arm all the way in to the center and do the same thing. Does it fall heavier either in or out? Both arms should react similarly. If the right falls in, place a jack under the brush, loosen the pivot bearing bolts slightly to slightly counter the direction of fall. Retighten the bolts and check again. This should provide a more balanced action of the rear arms closing.
