Whatever you do, do not get under it. Block it in position whenever you're not making an attempt to get it down. Determine which knob does what. One engages the PTO, the other moves the raise and lower valve. Next, determine if it's a genuine dump, single acting cylinder. Single acting cylinders have only one hose connected to them. Dual acting cylinders have a hose at each end. IF someone were to have fabricated their own dump hoist with a dual acting cylinder, then you must have the PTO engaged to lower the bed. A really dumb thing to do, but when that's what you have for parts, then that's what you use! If it's a standard dump hoist with single acting cylinder, locate the valve control lever. It has 3 positions. Raise, hold, and lower. Cable pulled is raise, center is hold, push is lower. Remove the cable end from the lever. Using the largest pair of channel locks you can get your hands on, or a pipe that will slide over the lever, move the lever from the center to raise position, so you have an idea how much travel to expect from the center position. Go back to center and then try pushing the lever beyond center to the lower position. Often times the lever will bind enough that the force of pushing on the cable is not enough to overcome the resistance of the lever. All it does is flex the cable housing, or just bends the cable between the housing and the lever. Cables are pull tools and not push tools, though most generally sturdy enough to move the lever to the lower position, IF it gets used fairly frequently. Once your bed is down, give that lever a good number of reciprocations until if feels relatively freed, then try it with the cable connected.
