I am the operator of an extremely old Yale forklift. The forks will tilt back but get stuck moving forward. We have removed and disassembled the shifter valve body and can find nothing apparently wrong with it. We have removed the tilting pistons and used air pressure to assure they aren't stuck. They work with air pressure but won't work on the lift. Up and down also works ok. We have removed the pressure lines and flushed them out. Power steering is a little harder than normal, but since it goes up and down we know we are getting hydraulic pressure from the pump. If a hydraulic line is removed from the tilt pistons they fall forward. Nothing has worked so far to get it to tilt forward without opening a line.