Copy Custom Brush To Another Open File?

Is there a way to use a custom brush made in one open file, in another separate open file? Seems like all custom brushes should persist within every open file in the program, but it seems that each open file has it’s own set of brushes, and you can’t transfer between them. Am I missing something? If you can’t do this, then this is a pretty major oversight. All I want to do is use one image to create custom brushes, and actually paint with them in a new separate image.