99% of the time you don't have to select or modify the texture of any brushes to fix cubemap issues.
All you need to do is plop an env_cubemap entity in the right place.
As a player runs around your map, the reflections they will see are generally based on whatever env_cubemap is nearest to them. At least to start out, don't go overboard with env_cubemap, you probably don't need more than one-per-room until you get towards the final release of your map.