Brake not working

Hello,

There are a few different reasons why the brake pedal can go to the floor.
It could be due to air in the brake lines, leaking brake fluid, faulty master cylinder, it can also be due to low fluid level.

Here is a helpful guide:

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Are you able to tell where the hissing noise is coming from?
Is it around the master cylinder?
Hissing noise is usually caused by air.

Let me know of any questions.

Thank you.