Redirect logout to PMPro login page.

logout_url_redirect_pmpro.php

<?php

// Copy from below here...

/*
 * Redirect logout to PMPro login page.
 */
function my_logout_url_redirect_pmpro( $logout_url ) {
	global $pmpro_pages;
	if ( ! empty( $pmpro_pages['login'] ) ) {
		$logout_url .= '&redirect_to=' . urlencode( get_permalink( $pmpro_pages['login'] ) );
	}
	return $logout_url;
}
add_filter( 'logout_url', 'my_logout_url_redirect_pmpro' );