To correctly using the admin variable to exclude admin visits from the Google Analytics data, please follow the following instructions provided by one of our users (patrick_bell).

Create an Exclude filter to remove data from visitors with this cookie. Follow these instructions to create a filter with the following settings:

Filter Type: Custom filter > Exclude
Filter Field: User Defined
Filter Pattern: admin
Case Sensitive: No

That's it. It should now be properly excluding the admin visits.