Hissing noise from brake pedal

So on my 66 Newport, I recently have a slight hiss coming from the brake pedal inside the car. It goes away when I apply the brakes. There is no rough idle or anything associated with a vacuum leak to the engine. I’ve had a brake booster fail before, but the symptoms it displayed were a loud hiss when the brakes were applied with a coinciding rough engine idle. This seems to be the opposite. Any thoughts on what part is failing? Thanks in advance.
Author: darth_linux