The head valve is one of the major components in a nail gun. When the trigger is depressed there is greater pressure below the head valve plunger than above it. The plunger rises up, and the compressed air makes its way to the piston head. When air pressure is greater above the head valve plunger than it is below the piston, air is released and air travels through the valve, forcing the piston/driver downwards to drive the nail/staple. There are o-rings inside and outside of the head valve. If the head valve leaks, try replacing the o-rings first, then the valve itself, as it will wear down over time.
This item fits the following models:
15-pc Thunderbolt Black Oxide Drill Bit Set
1/4" x 50 Ft Noodle Air Hose with Coupler and Plug