In sort selection, you are selecting a field (column) on which to sort your records. Yet from your description, "Brand" is one field in which their can be "about 20 brands". That sounds like a find, rather than a sort. If you select "Brand A" do you only want records with "Brand A" in the Brand field to appear on the layout?
Make Sort Brand a global text field. (Global storage is a field option you can select in Manage | database | fields.)
Use this script:
Enter Find Mode -->clear the pause check box
Set Field [Assets::Brand ; Assets::Sort brand ] --> this step will not work unless Sort Brand has global storage specified.
Set Error Capture [on]
Perform Find 
Perhaps "select brand" would be a better name for this.
Thanks Phil. this worked perfectly!!