See how far the adjusting arm will move with air pressure (80 psi or above)or by manually turning the arm with a pry bar. It should not go beyond the 90 degree point illustrated in figure B. You may or may not have the visual over-stroke indicator on the pushrod that is shown here. At very least, adjust the slack until the stroke does not go beyond the 90 degree mark. Ideally you should bring the free play in the arm down to 3/8" and the adjusting arms should all be at the same angle with the same pressure applied. Don't overtighten. Bang the brake drum with a hammer with the pressure off. The drum should ring hollow if the shoes are clear of the drum. A dull thud means you still have lining-drum contact and you should back off the adjustment. When you bring the free play down to about 3/8'', the stroke should be well short of the 90 degree point.