problem w/ sessions

Hi folks.
I'm kinda new at this, but I'm having trouble retrieving (or setting) Session variables.  They seem to be lost whenever the page reloads.
Aren't they supposed to remain until unset or until the browser is closed?

brief summary of what i'm doing:

at the top of the page:

if(!isset($_SESSION)) {
      session_start();
      header("Cache-control: private");
}

if ($action == "logout") {
      $_SESSION = array();
                session_unregister();
      session_unset();
}


if ($action == "login") {
                $_SESSION['login'] = 1;
}

if ($_SESSION['login'] ==1) {
                stuff;
} else { other stuff;  }


Now, I don't think I'm properly checking whether or not the session is set.  Basically, I don't want it to be destroyed until the user logs out or closes the browser window.

Am I using the Session functions incorrectly?
Is there a better way to check whether a user is logged in or out in this scenario?