Setting form field to required with custom directive

I have a directive that queries google maps api with the form field value (event_postcode). It needs to be a required field. Yet when the field is populated, and maps api (via manageEventsSrv.getPostcodeLocation()) returns a valid address, the forms submit button remains disabled, i.e. form is invalid.

My directive:

app.directive('geoLocatePostcode', ['manageEventsSrv', function(manageEventsSrv) {
    return {
        restrict: 'A',
        require: 'ngModel',
        link : function(scope, elem, attrs, ctrl){
            var valid = false;
            ctrl.$parsers.unshift(function(value) {
                if(value.length>=3){
                    manageEventsSrv.getPostcodeLocation(value+',uk').then(function(result){
                        valid = true;
                        scope.point = new Parse.GeoPoint({latitude:result.lat,longitude:result.lng}); // parse.com geopoint
                        ctrl.$setValidity('locationNotFound', true);
                    }, function(result){                            
                        valid = false;
                        ctrl.$setValidity('locationNotFound', false);
                    });
                } else {
                    ctrl.$setValidity('locationNotFound', true);
                    valid = false;
                }
                return valid ? value : undefined;
            });
        }
    }
}]);

Form field:

<input type="text" placeholder="First 4 Characters Only" ng-model="event_postcode" maxlength="4" name="event_postcode" required geo-locate-postcode>

<span data-ng-show="createEventForm.event_postcode.$error.locationNotFound">Location not found</span>

<button type="submit" class="button" ng-disabled="createEventForm.$invalid">Create</button>

I assume i am not implementing "required" properly when using a custom directive.