the mechanic is telling me I need a new brake master

Hello, I'm Danny.

Here is the information you requested. Here are a few tutorials for you to view on how to replace a brake master cylinder and how to bleed the system after the repair has been made:

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

Since you are installing a new brake master cylinder it would be best to bench bleed the unit before installation to remove air from the new unit prior to installation on the vehicle. Here is a YouTube tutorial from our site showing how to do so:

https://youtu.be/WDxvEQrMkBg

I've attached picture steps below on how to replace the brake master cylinder and bleed the system on your car. Hope this helps and thanks for using 2CarPros.