So the shoes move when you pull the lever on, but have you tried to pull the drum off while the brakes are pulled full on? Perhaps the shoes aren't contacting the drum firmly enough, and have you had the shoes radiused to the drum (assuming you may have replaced the shoes)? If you can pull them on & the truck still rolls, I'd be looking at the contact between the shoes & drum. You can bet it will be something simple. Good luck.
