When I need to flash old inputs I do it like this

    public function postSubmitAction()
    {
        $form = new MyForm();
        if($this->request->isPost())
        {   
            $model = new MyModel();
            if($form->isValid())
            {
                //save
            }
            else
            {
                //saving input values to session
                 foreach($this->request->getPost() as $key=>$msg)
                {
                    $this->persistent->$key = $msg;
                }
                return $this->response->redirect('/submit');

            }
        }
    }

    public function getSubmitAction()
    {   
        $form = new MyForm()
        //if there are any persisten values show them and remove after they are shown
        if($this->session->has(__CLASS__))
        {
            \Phalcon\Tag::setDefaults($this->session->get(__CLASS__));
            $this->session->remove(__CLASS__);
        }
        $this->view->setVar('form', $form);
    }