to draw it toward the rear frame member to raise the tube means and also to allow the upper ends of the frame members to move away from each other to lower the tube means the flexible cable means are attached at the other end to hydraulic cylinder means having head and rod ends and reciprocable piston means having associated piston rod means and operatively associated hydraulic line means attachable to vehicle hydraulic system means the hydraulic cylinder means have front triple sheave means attached to the piston rod means and rear triple sheave means adjacent the head end of the hydraulic cylinder means the flexible cable means passes from the upper end of the front frame member around the rear sheave means then around the front sheave means to anchor means rearward of the hydraulic cylinder means whereby extension of the cylinder piston means draws the front frame member toward the rear frame member the improvement provides the piston means including checkvalve means the checkvalve means preventing flow through the piston means from head side to rod side allowing head side hydraulic pressure to extend the cylinder and allowing flow from rod side to head side preventing rod side hydraulic pressure retracting the cylinder whereby.