C# reset wpf toolkit CheckComboBox

I have a CheckComboBox from the wpfToolkit and i want to reset it. That means that all items in the CheckComboBox are unchecked. Already tried to set the SelectedValue to an empty string. This works for the function itself, but in the UI the items are still checked.

I also work with Bindings and the MVVM pattern:

 <wpfTool:CheckComboBox IsSelectAllActive="True"  ItemsSource="{Binding Filter, UpdateSourceTrigger=PropertyChanged}" SelectedValue="{Binding SelectedFilterItem, UpdateSourceTrigger=PropertyChanged}" Width="140"/>