MVC custom unobtrusive validation always showing message
$.validator.addMethod('newserialandmodel',
    function (value, element, parameters) {
        var modelNumber = $('#ProductInformation_ModelNumber').val();
        var serialNumber = value;

        var token = $('input[name=__RequestVerificationToken]').val();

        $.ajax({
            type: "POST",
            url: "/ajax/getmodelandserialexists/",
            data: ({
                    modelNumber: modelNumber,
                    serialNumber: serialNumber,
                    '__RequestVerificationToken': token
                  }),
            cache: true,
            dataType: 'json',
            success: function (response) {
                if (response.exists) {
                    $.validator.messages.newserialandmodel =
                        $.format($.validator.messages.newserialandmodel);
                }

                return !response.exists;
            }
        });

        return false;
    }
);

$.validator.unobtrusive.adapters.add(
    'newserialandmodel',
    function (options) {
        options.rules['newserialandmodel'] = options.params;

        if (options.message != null) {
            $.validator.messages.newserialandmodel = options.message;
        }
    }
);

<input type="text" value=""
 name="ProductInformation.SerialNumber" id="ProductInformation_SerialNumber"
 data-val-required="The Serial number field is required."
 data-val-newserialandmodel="There is already a contract with the provided model and serial numbers."
 data-val-length-max="30"
 data-val-length="The 30 field requires no more than 30 characters."
 data-val="true" class="input-validation-error">

$.validator.addMethod('newserialandmodel',
    function (value, element, parameters) {
        var modelNumber = $('#ProductInformation_ModelNumber').val();
        var serialNumber = value;

        var token = $('input[name=__RequestVerificationToken]').val();

        var isValid = false;

        $.ajax({
            type: "POST",
            url: "/ajax/getmodelandserialexists/",
            data: ({
                    modelNumber: modelNumber,
                    serialNumber: serialNumber,
                    '__RequestVerificationToken': token }),
            async: false,
            dataType: 'json',
            success: function (response) {
                if (response.exists) {
                    $.validator.messages.newserialandmodel =
                        $.format($.validator.messages.newserialandmodel);
                }

                isValid = !response.exists;
            }
        });

        return isValid;
    }
);