[Plugin: Event] Adding sortable custom columns to admin screen

This plugin is currently showing the custom column event_date just fine but I’d like to be able to sort it on the WordPress admin back-end like the other columns that already exist, (Title and Date).

This is for the custom post type event. I thought this could be handled with the add_filter for manage_custom-post-type_sortable_columns function. What am I missing?

<?php
/**
 * Plugin Name: Event
 * Description: Event Schedule.
 * Version: latest
 * Author: Infoway
 * Author URI: http://URI_Of_The_Plugin_Author
 * License: GPL2
 */
?>
<?php
ob_start();
global $table_prefix;
define('EVENT_PLUGIN_PATH',  dirname(__FILE__));
define('EVENT_PLUGIN_URL',  plugin_dir_url(__FILE__));
define('EVENT', 'event');
define('EVENT_CAT', 'event_cat');
/*TBL Define*/
define('LOCATION_TBL', $table_prefix.'location');

register_activation_hook(__FILE__, 'event_actvation');

add_filter('manage_'.EVENT.'_posts_columns', 'custom_column_name_func',10,2);
add_action( 'manage_'.EVENT.'_posts_custom_column' , 'custom_column_value_func', 10, 2 );

function custom_column_name_func($columns) {
    unset($columns['date']);
    $columns['event_date'] = 'Event Date';
    $columns['date'] = 'Date';
    return $columns;
}

function custom_column_value_func($columns,$post_id){
    $event_date = get_post_meta($post_id, '_ev_start_date', true);
    switch ( $columns ) {
        case 'event_date':
            echo date('F j, Y', $event_date);
        break;
    }
}

function custom_column_sort($columns) {
	$custom = array(
		'Event Date' 	=> 'event_date'
	);
	return wp_parse_args($custom, $columns);
}
add_filter("manage_'.EVENT.'_sortable_columns", 'custom_column_sort');

function custom_column_orderby( $vars ) {
	if ( isset( $vars['orderby'] ) && 'event_date' == $vars['orderby'] ) {
		$vars = array_merge( $vars, array(
			'meta_key' => 'event_date',
			'orderby' => 'meta_value'
		) );
	}
	return $vars;
}
add_filter( 'request', 'custom_column_orderby' );