knockout binding is not working for enable button after ko.applyBindings in bootstrap modal

I'm trying to enable a button after check a checkbox, the elements are in a modal. I'm using MVC, and I'm adding the observables after the main binding for use them just with the modal like this

<script type="text/javascript">
    var Model = function () {
        self.check = ko.observable(false);   
    };

    $(document).ready(function () {
        ko.cleanNode($('#Modal')[0]);
        ko.applyBindings(Model, $('#Modal')[0]);
    });

</script>

The html elements inside the modal look like this:

<input type="checkbox" data-bind="checked:viewModel.check">bla bla..
<button type="button" data-bind="enable:viewModel.check==true" class="btn btn-primary">Delete</button>

when I select the checkbox the viewModel.check is true and when is not checked is false which is working fine but the button is always disable. Any advise please