If your vehicle is showing hissing noise from under the dashboard when pressing the brake pedal, here are the most likely causes and the parts you'll need to fix each one.
The brake booster uses engine vacuum to assist your braking. If the internal rubber diaphragm tears or the atmospheric valve fails, it creates a vacuum leak that hisses loudly into the cabin when the pedal is pressed.
A rubber boot and foam silencer surround the rod connecting your brake pedal to the booster. If this boot tears from age or wear, the normal rush of air during braking is no longer muffled and becomes an audible hiss.
The check valve or its rubber grommet on the booster can crack from heat or get knocked loose during under-hood maintenance. Though the leak is in the engine bay, the hissing sound easily travels through the firewall to the dashboard area.