How do I adjust the back brakes? The pads do not close onto the rim at the same time.

The calipers have an internal recoil spring on each side. The small screw on the side near the pivot mount increases or relieves the tension. A clock-wise turn pulls that side away from the rim, while a counter-clockwise turn on the opposite side will help pull the closer pad away from the rim. Adjust each at the same time in half-turn increments, testing the brake pull after each adjustment. Usually the pads can be centered evenly after 2.5 to 3 turns. Neither side should be full-turned to its maximum in or out. Lubricate the pivot mount with a little WD-40 and insure the brake cable is not binding.