To identify what part needs replacement, understanding the parts is important. The black line running from the hand brake to the brake pad on the wheel is a “cable cover”. The actual brake cable runs inside this cover. If the cover needs to be replaced the ends will usually fray exposing little steel wires that will prick your fingers. At the wheel the brake pad is only half of the process as the wheel must have rubber to meet the brake pad. If a back wheel is worn to the point that the pad and the wheel do not meet you might try to switch the front wheels to the back if the front wheels have more rubber. On the backside of the brake handle is a round little knob about ½” in diameter and ½” sticking out. This looks to be part of the brake but is actually where the brake cable feeds from. Grab this with a pair of pliers and it will come right off. After the new cable is fed through the cable cover make sure the cable adjustment screws at both ends are returned back to a “no adjustment” position. (If the adjustment screw is screwed out this lengthens the cable and tightens the brake. The little nut is finger tight only!) It helps to have a second set of hands when setting the brake for the first time actually keeping the brake pads closed against the wheel or down on the wheel, depending on the type of brake pad. This takes the slack out of the cable so the brake has tension to the wheel. Rule of thumb is not to have the brake locking mechanism too tight so it would be difficult to set. The brake locking mechanism is achieved by pressing down on the brake handle and it should click into the lock position and not be too hard to press down. If it does not hold this is an indication of the need for a new brake.