change your code into:
private ObservableCollection<MLBPage> _PageCollection = new ObservableCollection<MLBPage>();
public ICollectionView _PageCollectionView { get; set; }
just do this once (eg. within ctor)
//ctor
_PageCollectionView = CollectionViewSource.GetDefaultView(_PageCollection);
_PageCollectionView.Filter = FilterLeadersList,
use clear, add , remove to alter your _PageCollection.
bind your listbox to your view
<ListBox ItemsSource="{Binding _PageCollectionView}"/>
use Refresh to refresh your filter
_PageCollectionView.Refresh();