filtering custom post types via meta data drop down

Because you want the results of the filtering process be the main object of your page, you may need to alter the main query using pre_get_posts action hook. It is really better that leave the main query run, load your theme and then say to WordPress: go back and run another query as suggested in other answers.

//Functions for filters
add_action( 'pre_get_posts', 'my_pre_get_post' );
function my_pre_get_post($query){

     //limit to main query, frontend and archive pages. You need another set of checkings, like check if the request is for your custom post type
     if($query->is_main_query() && !is_admin() && $query->is_archive ) {

          $meta_query = array();
          $meta_input = isset($_POST['meta_input']) ? $_POST['meta_input'] : '';

          if(!empty($meta_input)){
              $meta_query[] = array(
                  'key'  => 'your_meta_key',
                  //sanitize $_POST['meta_input'] according with your data type
                  'value'    => $_POST['meta_input'],
              );
          } 
          $query->set('meta_query',$meta_query);
    }

}

See more information about the meta_query.

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