Trigger valve for pneumatic nailing machine

A trigger valve for a pneumatic nailing machine comprising:

• a head valve which is provided in a passage between a main air chamber and a main cylinder in which a driver piston reciprocates, and which opens and closes said passage;
• a trigger valve which is incorporated in an air passage communicating with a chamber above said head valve and with said main air chamber, said trigger valve forming a trigger valve cylinder comprising two cylinder parts that divide it into an upper portion and a lower portion said upper and lower portions being closely joined together;
• a lateral hole formed in the upper cylinder part of said cylinder, which communicates with said main air chamber and with said air passage;
• an exhaust hole formed in the lower cylinder part, which communicates with the atmosphere on the side of the trigger lever;
• a cylindrical trigger pilot valve stem provided in the trigger valve cylinder, which is slidable up and down within a range bounded by top dead center and bottom dead center;
• first, second and third O-rings fitted onto said trigger pilot valve stem to open said lateral hole and said main air chamber when said stem is located at top dead center and to close said lateral hole and said exhaust hole when said stem is located at bottom dead center;
• a small-diameter communication hole formed in an upper closed surface of said trigger pilot valve stem, which communicates with a hollow portion in said stem and with said main air chamber;
• a pilot air chamber which is formed in said lower cylinder part and which communicates with said main air chamber via said small-diameter communication hole;
• a trigger valve stem provided in the lower cylinder part, which is slidable in the hollow portion formed in said trigger pilot valve stem;
• and fourth and fifth O-rings which are fitted onto said trigger valve stem to open and close said small-diameter communication hole and said pilot air chamber.