Let me quote from warpig.com -
-- The Valve Stop goes in a slot under the receiver, in front of the trigger guard just behind the velocity adjustment allen screw. The Valve Stop is usually held in place by the front pin that holds the trigger assembly to the receiver. The Valve Stop itself prevents the main valve body from moving back and forth in the receiver.
Now, does anyone have an alternative to replacing this, because I cannot for my life find one online.