It sounds like you've got a lift valve malfunction of some sort. You should be able to raise the bed a little, and go to the "hold' position in the center of the lever's travel without disengaging the PTO. Of course, there are all sorts of lift systems out there, and I've by no means worked with all of them. It just doesn't make good nonsense to have an "all or nothing" situation when lifting the bed. Hydraulic pumps have a relief valve built in to prevent damage when running against a closed valve, but it's not a good idea to do that for long. All the drivers I worked with at a big sand, gravel, and concrete outfit in California knew to disengage the PTO before getting back onto the road if they wanted to stay employed!
Jerry
