How to disable redirection after login_check in Symfony 2

I need to disable redirection after login check, because I need to get only that the login was success or not. After submission /login_check url give me the right data, but keep redirecting to /login (on failure).
/login is blank after that.
I am trying to set up login form using extjs 4 so I need to validate trough an ajax post request. login_check should authenticate, create user session and return whether it was success or failure, but no forwarding anywhere.

my login.html.twig looks like:

{% if is_granted("IS_AUTHENTICATED_REMEMBERED") %}
    { success:true }
{% else %}
    { success: false }
{% endif %}

and in security.yml:

firewalls:
    main:
        form_login:
            provider: fos_userbundle
            failure_path:  null
            failure_forward: false