addfilter order problem

I want to show all my post in mt rss feed,
so I can not filte <!–more–> in my plugin,why?
the php code is listed as bellows

function kill_more_for_feed($data) {
//this is the first version
//you can set up to 10 ads type
require_once("googleads.php");

	//find the google ads to replace

    if( is_feed()  )
	$data = str_replace('<!--more-->', '', $data); 

    return $data;
} 

add_filter('the_content', 'kill_more_for_feed');

but it does not work,how can I do it,thanks!