TM 10-3930-653-14&P (3) Operation of Tilt Lock Valve The  tilt-lock  valve  is  intended  to  prevent  vibrations  of  the  mast  due  to  possible  creation  of  internal  negative pressure in the tilt cylinder and also to avoid danger incurred from mis-handling of the lever.  On the conventional model, even if the engine is off, the mast can be tilted forward by actuating the tilt lever.  But this newly adopted tilt-lock valve  does not allow the mast to tilt forward as long as the engine is shut off, even if the tilt lever is pushed with the full load.  Refer to Fig.  16-9 for the construction of the tilt-lock valve. The port " A" side of the plunger housing is led to the front side of the tilt cylinder, and the port "B" side to its rear side.  When the tilt lever is pulled backward (plunger drawn out), the oil from the pump flows into the port "A" while the port "B" side oil returns to the tank, so that the mast is tilted backward by the tilt cylinder. When the tilt lever is pushed forward (plunger pressed), the oil from the pump flows into the port "B".  But in the port "A" side, oil does not return to the tank, unless the poppet installed in the plunger is moved.  In other words, the mast would not tilt forward unless the oil pressure at the rear side of the tilt cylinders reaches a certain level.  Hence, while the engine  is  being  shut  down,  the  mast  never  tilts  forward,  nor  does  the  internal  pressure  in  the  tilt  cylinders  become negative. Fig.  4-474.  Tilt Lock Valve 4-238