acls not working?

I just tried to get into the magic of node access modules, so I had a look at the acl and forum_access modules. However, I wondered if an implementation of hook_node_grants() isn't necessary for the acl module? form_access does just return the rid..

So I tried if the acl does really work. I removed edit post access from every roles but added a moderator. Then I created a new post in this forum, which was editable by the admin user. Then I logged in as moderator, but I wasn't able to edit the post.
So I'm wondering if the acls are correctly applied?