You may also need the following directive to be set before the first "sub_filter" for backend-servers with data compression:
proxy_set_header Accept-Encoding "";
Otherwise it may not work. For your example it will look like:
location /admin/ {
proxy_pass http://localhost:8080/;
proxy_set_header Accept-Encoding "";
sub_filter "http://your_server/" "http://your_server/admin/";
sub_filter_once off;
}