Get Boolean Value for a checkbox

Hello I have a checkbox in my page and I want to get a X value from the backEnd but the checkbox only take a boolean for value. I tried on my formatter to code this :

changeValueBox: function (sBreakDown) {
                switch (sBreakDown) {
                case "X":
                    return false;
                default:
                    return true;
                }
        }

And on my view I got this :

    <CheckBox valueState="{path: 'maintenanceOT>BreakDown', formatter: '.formatter.changeValueBox'}" />

But i got a problem with the "valueState" I don't know what to write otherwise. Thank you for your help.