John, I always lay a hardwood block across the frame, just in front of the hinges. Approx..4"x6"x4'. If you do this, test it by letting the box down on it to make sure it'll hold. As mentioned before never work under a loaded box!! You probably have a "stop" cable attached to the valve in front, and the back of the box in the rear. This cable will stop the hoist from overextending, and should also pull the valve into neutral. Do not hit this cable when the box is up!! This WILL bring the box down! Most new trucks have air controlled hydraulics, thus eliminating this cable. Jeff.
