Well I can't say for sure because I'm not there but a hissing sound when you release the pedal is a leak in your brake booster. If it's grinding or squealing while your pressing on the pedal then it's your pads. If the pedal is going all the way to the floor it's your brake master cylinder. If it's the booster or the master cylinder it will require some experience to change it since these systems on your car are THE MOST IMPORTANT since they make your car stop. I personally recommend taking it in. If you absolutely can't (don't have the money to) you must diagnose the problem further to insure you don't buy a part that you don't need and research thoroughly online on how to replace it and attempt it. Make sure you test it adequately before taking it at normal road speeds. Comment me if you have any more information on what it does and sounds like when you press on the brake pedal.
If your brakes are hissing and not stopping, then that means that they need to be replaced with new brakes or brake pads. You can always go to the store and by the brake pads and put them on yourself. It may also mean that your brake fluid is low.
The hissing, or screeching noise you here is from your pads making contact with the brake rotor. The longer you drive with this issue the more you damage your disc rotors, meaning that you might have to get them shaved or if it's really bad to the point that they have worn out you gonna need to replace them. If you have the machine at home to do this I advice you to replace the pads and shave the rotors, but if you don't, replacing the pads only won't help. The noise might go away for a while but you may experience shaking in the steering wheel when you step on the brakes due to the unevenness of the brake rotors. Hope this was helpful.