Hi Kent,
Have you got the rest of the model No of this truck,
Is it a GDP060 AB/TD/AF is it quite an old truck, dose it have a valve block screwed to the cross member at the back of the mast, if it has then there's a good chance that this valve has failed, I have experienced the cone shaped piston in this valve breaking up. Looking in an old w/shop manual this valve is referred to as the lowering valve but is not part of the main control valve.