ExtJs获取后台返回值的问题

ExtJs新手，碰到个问题：获取后台传的Json的值解析不了：具体看代码注释部分：
前台代码：非关键代码省略了
........
 Ext.Ajax.request({
                    url: 'RequireLeave/DoRequireLeave',
                    method: 'POST',
                    waitTitle: '表单提交',
                    waitMsg: '请等待...',
                    success: function(form, action) {
                        var obj = Ext.util.JSON.decode(form.responseText);
                        alert(obj);//这里弹出来的值是{success:true}                        //但obj.success却没有值，直接走到else里去了
                        if (obj.success == true) {
                            Ext.Msg.alert('提示', 'OK');
                        }
                        else {
                            Ext.Msg.alert('提示', "2" + obj.success + "1");
                        }
                    },
                    failure: function(form, action) {
                        Ext.Msg.alert('警告', '系统错误');
                    }
                });
...........后台关键代码：
............
 public ActionResult DoRequireLeave(string epName, string beginTime, string endTime, string reason)
        {
            return Json("{success:true}");
        }
.............