Raycast Coding Issues

this code is suppost to test if something is below the test point. It does this ONCE. How do I get it to reset?

function Update ()
{
var hit : RaycastHit;

if (Physics.Raycast (transform.position, -Vector3.up, hit, 3) != null) 
 {     	
 	SendMessageUpwards ("conector", true);
 }
 else
 {
 	SendMessageUpwards ("conector", false);
 }

}

Physics.Raycast () doesn’t return a nullable type. It returns boolean. Check if the result is true or false rather than null.