Add a "read more" link after both manual excerpts and regular excerpts and customize the link text for custom post types in WordPress.

functions.php

<?php
//* Add read more link after both manual excerpts and regular excerpts and customize link text for custom post types
add_filter('the_excerpt', 'amn_manual_excerpt_read_more_link');
function amn_manual_excerpt_read_more_link($output) {
  global $post;
	if ( get_post_type() == 'my_custom_post_type_slug' ) {
		$link_text = 'My CPT Read More Text';
	} else {
		$link_text = 'Read More';
	}
	return substr($output,0,-5).'... <a href="' . get_permalink($post->ID) . '">' . $link_text . '</a>.</p>';
}