Custom post_type search’s $wp_query->query_vars do not correspond

build the url parameters something like this :

?post_type[]=event&post_type[]=feature&post_type[]=brief

Then $searchposttypes = $_GET['post_type'] will have an array. pass it on to the wp_query

i should add that you should validate the $_GET parameters before going on with it.

reference : wp_query – type parameters