The following documentation refers to 7.x-1.1 version and above.
The Read More Control module provides complete control of the Read More link, allowing you to append this to nearly any view mode and to any entity type, as well as customizations to the link inserted.
Firstly, a basic overview of some core Drupal terms. In the Drupal world, standard pages / content items are entity types called nodes. Other entity types include; terms, users, etc. Each content type is the bundle, for example the article and page types from the standard install. The custom display settings are controlled by the view modes; full content, teaser, search index, etc.
By default, nothing should change when you enable this module. The default behaviour is to "Always show (the Read more) link". However with this module, you have the option to configure this per entity type or bundle or even per view mode.
It is best to configure this globally and update individual bundles / view modes as required.
Global configuration is set here: Administration » Configuration » Content authoring » Read more settings.
You may see more than one tab if more than one entity type can support this settings. Node should be the default.
Restoring Drupal 6 behaviour - Show Read More if body field has more content.
Go to the global configuration options:
Administration » Configuration » Content authoring » Read more settings.
Select the "Show link when required by the Body (body) field".
Save and the standard Drupal 6 behavior should be restored on your site.
Changing the Read More display on a specific content type
To override the Read more link on targeted bundles or view modes, go to the entity types display settings. So for the Page content type, go to:
Administration » Structure » Content types » Page » Manage display
This clones the global configuration options, with an additional option, "Inherit default behaviour". Select the behaviour that you want to enforce and save. You could repeat this per view mode if so desired.
It is best to update the default view mode and have these settings inherited by the child view modes.
Enabling a Read More on additional view modes
To block this link from showing everywhere by default, you actually have to configure the setting to show on non-teaser displays. Until the display settings are saved, there is a safeguard switch in place to hide the link.
Assuming that the global is set to show and this is not being overridden by the default view mode, visit the display that you want to enable and simply save. The default behaviour should be "Inherit", and once saved this setting will be used.
To disable, simply revisit this page and select "Never show link" and save.