How would I clear values in ajax loaded select options?
SOLVED

/**
 * The ajax call to return data for the printer model select.
 * @return array             An array of settings.
 */
function first_ajax_callback(&amp;$form, &amp;$form_state) {
  // Reset conditional select values.
  $form['field_printer_model_value']['#value'] = array('All' =&gt; 'Select Printer Model');
  $form['field_printer_type_tid']['#value'] = array('All' =&gt; 'Select Device Type');

  // $commands = array();
  $commands[] = ajax_command_replace("#second_dropdown", drupal_render($form['field_printer_type_tid']));
  $commands[] = ajax_command_replace("#third_dropdown", drupal_render($form['field_printer_model_value']));

  return array('#type' =&gt; 'ajax', '#commands' =&gt; $commands);
}


In the above call I set the #value of the selects I want to reset. These lines properly reset two later dependent selects:

$form['field_printer_model_value']['#value'] = array('All' =&gt; 'Select Printer Model');
$form['field_printer_type_tid']['#value'] = array('All' =&gt; 'Select Device Type');