You need to prevent the submit (event) of the form with .preventDefault()
so it will not do the default behaviour: submit the form and reload the page.
$('.btn.btn-large.btn-primary').click(function(event) {
//if only button on page you can have just $('button')
event.preventDefault()
LoginCheck();
})
You can also add type="button"
if you do not want to use the button
as a submit button, because the button element has a default type of submit:
<button type="button" class="btn btn-large btn-primary">Sign in</button>
You can read more here:
jQuery docs: .preventDefaul()