brake air leak (1993 International 9400-Series 60)

When I step on my brake pedal, I get a rapid air leak the harder I press the pedal. I located the air hiss, and it’s coming from outside under the hood coming out of the firewall. It is a small plastic tube that goes no where. I thought either it was a broken tube, but I don’t see where it could go to. Or I thought it could be some sort of breather tube. But that don’t make sense to me either. I know I shouldn’t be losing air from it when I’m pressing the brake. If I can figure out how to upload a picture of it, maybe someone can tell me why it’s leaking brake air. I assume it would not be wise to plug it. Even though it does not leak anymore if I plug it. But I don’t know if that will affect any other part of my air system or braking. Any ideas?