My 1943 Farmall H tractor lift-all system has 850 psi of pressure. When I pull the lift-all lever out the hydraulic cylinder attached to the system extends, when the lever is pushed back in it will not come back down even with a heavy weight on the unit. The only way I can get the cylinder to come down is disconnect the hydraulic line, releasing oil and pressure. I am guess the problem is a stuck relief valve in the internal lift-all system. I have tried connecting the hydraulic hose to other ports on the system, but the same results. I have tried pulling the lever constantly, the pressure goes up to 850 psi and when I push the lever in the psi drop to 600 and then later 400 and eventually to 200. Is there a way of releasing a stuck valve in the lift-all system without dropping and opening up the lift-all system? Help.