Pullstarters are notoriously sensitive. Try not to pull them out to full extent, use short, quick pulls. They are not mechanically complicated, I suggest you take the engine out and take the pullstarter assembly off the back of it, and inspect it. The only thing that is a pain is if the coil spring inside comes out. They can be put back but it isn't easy. IF the worst comes to the worst, a new pullstart is not expensive either, though I don't know where you would get one for that truck.