The valve is located about mid way inside the lower tube.
In order to get to it, you need to remove the grip frame.
Now look at the bottom of the body, and youll see a copper colored screw.
You need to unscrew that...its called the valve retention screw.
Once thats out. unscrew the plug/low pressure chamber at the front of the lower tube...under the barrel.
Now allow the valve pin assembly to fall out...carefull to keep the spring and all in one piece.
Look inside the lower tube.
That silver contraption inside is the valve.
Use a pencil with a good erasure end, and gently push the valve out the back of the lower tube.
Be sure to lube both O-rings before reinserting the valve through the front of the lower tube...and take note that one of the holes found between the O-rings is smaller than the other.
Thats the one that the valve retention screw needs to marry up with.
Also...when reinserting the valve...the black O-ring goes in first.